test: cmd: fdt: Test fdt memory

test: cmd: fdt: Test fdt memory

Add 'fdt memory' test which works as follows:
- Create custom FDT with /memory node, with select #*cells, map it to sysmem
- Perform memory fixup
- Read back the /memory node and validate its content

The test case can be triggered using:
"
./u-boot -Dc 'ut fdt'
"
To dump the full output from commands used during test, add '-v' flag.

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
Reviewed-by: Simon Glass <sjg@chromium.org>