docs: Add a fu540 document.

Add a readme guide for fu540 with different types of build & booting steps.

Signed-off-by: Atish Patra <atish.patra@wdc.com>

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+SiFive FU540 SoC Platform
+==========================
+The FU540-C000 is the world’s first 4+1 64-bit RISC‑V SoC from SiFive.
+The HiFive Unleashed development platform is based on FU540-C000 and capable
+of running Linux.
+
+To build platform specific library and firmwares, provide the
+*PLATFORM=sifive/fu540* parameter to the top level `make` command.
+
+Platform Options
+----------------
+
+As hart0 in the FU540 doesn't have an MMU, only harts 1-4 boot by default.
+A hart mask i.e. *FU540_ENABLED_HART_MASK* compile time option is provided to
+select any other hart for booting. Please keep in mind that this is not
+platform wide option. It can only be specificd for FU540 platform in following way.
+
+```
+make PLATFORM=sifive/fu540 FW_PAYLOAD_PATH=Image FU540_ENABLED_HART_MASK=0x02
+```
+This will let the board boot only hart1 instead of default 1-4.
+
+Booting SiFive Fu540 Platform
+-----------------------------
+
+**Linux Kernel Payload**
+
+Build:
+
+```
+make PLATFORM=sifive/fu540 FW_PAYLOAD_PATH=<linux_build_directory>/arch/riscv/boot/Image
+```
+
+Flash:
+The generated firmware binary should be copied to the first partition of the sdcard.
+
+```
+dd if=build/platform/sifive/fu540/firmware/fw_payload.bin of=/dev/disk2s1 bs=1024
+```
+
+**U-Boot Payload**
+
+Note: U-Boot doesn't have SMP support. So you can only boot single cpu with non-smp
+kernel configuration using U-Boot.
+
+Build:
+
+The commandline example here assumes that U-Boot was compiled using sifive_fu540_defconfig configuration.
+```
+make PLATFORM=sifive/fu540 FW_PAYLOAD_PATH=<u-boot_build_dir>/u-boot.bin FU540_ENABLED_HART_MASK=0x02
+```
+
+Flash:
+The generated firmware binary should be copied to the first partition of the sdcard.
+
+```
+dd if=build/platform/sifive/fu540/firmware/fw_payload.bin of=/dev/disk2s1 bs=1024
+```
+U-Boot tftp boot method can be used to load kernel image in U-Boot prompt.
+
+Note: As the U-Boot & Linux kernel patches are not in upstream it, you can cherry-pick from here.
+
+U-Boot patchset:
+https://lists.denx.de/pipermail/u-boot/2019-January/355941.html
+
+Linux kernel patchset:
+https://lkml.org/lkml/2019/1/8/192
+
+**U-Boot & Linux Kernel as a single payload**
+
+A single monolithic image containing both U-Boot & Linux can also be used if network boot setup is
+not available.  
+
+1. Generate the uImage from Linux Image.
+```
+mkimage -A riscv -O linux -T kernel -C none -a 0x80200000 -e 0x80200000 -n Linux -d \
+               <linux_build_directory>arch/riscv/boot/Image \
+               <linux_build_directory>/arch/riscv/boot/uImage
+```
+
+2. Create a temporary image with u-boot.bin as the first payload. The commandline example here assumes
+that U-Boot was compiled using sifive_fu540_defconfig configuration.
+```
+dd if=~/workspace/u-boot-riscv/u-boot.bin of=/tmp/temp.bin bs=1M
+```
+3. Append the Linux Kernel image generated in step 1.
+```
+dd if=<linux_build_directory>/arch/riscv/boot/uImage of=/tmp/temp.bin bs=1M seek=4
+```
+4. Compile OpenSBI with temp.bin (generated in step 3) as payload and single hart enabled.
+```
+make PLATFORM=sifive/fu540 FW_PAYLOAD_PATH=/tmp/temp.bin FU540_ENABLED_HART_MASK=0x02
+```
+5. The generated firmware binary should be copied to the first partition of the sdcard.
+
+```
+dd if=build/platform/sifive/fu540/firmware/fw_payload.bin of=/dev/disk2s1 bs=1024
+```
+6. At U-Boot prompt execute following boot command to boot non-SMP linux.
+```
+bootm 0x80600000 - 0x82200000
+```