ARM: vexpress_ca9x4: Reintroduce board in order to use with QEMU.

ARM: vexpress_ca9x4: Reintroduce board in order to use with QEMU.

vexpress_ca9x4 is seemingly the only board except for qemu_arm which
is able to run U-Boot correctly, using the `-M vexpress-a9` option to
QEMU. Building for qemu_arm and running qemu-system-arm with the `-M
virt` argument has a number of downsides, most importantly that it
only supports virtio storage drivers. This significantly reduces its
usefulness in testing memory card and Flash solutions, especially when
the tested images are from a third party source.

So therefore we reintroduce the vexpress_ca9x4 board in this commit,
with the explicit goal of using it with QEMU.

A number of differences to note from the original:

* Since the board was apparently unmaintained, I have now set myself
  as the maintainer.

* The board has been converted to use the driver model, which was the
  reason it was removed in the first place.

* The vexpress_ca15_tc2 and vexpress_ca5x2 boards, which were removed
  in the same commit, are not necessary for the QEMU use case, and
  have been omitted.

* An `mmc0` alias was introduced in the dts file. The mmc is not
  detected correctly without this, now that it's based on the device
  tree instead of the board's init function.

* A couple of other nodes were removed because they were problematic
  when trying to run the UEFI bootmgr. Once again, the primary use
  case here is QEMU, and these nodes are not needed for that to work.

* Unnecessary board init code has been removed, thanks to driver model
  and device tree.

* `CONFIG_OF_EMBED` has been enabled. I know this goes against
  recommended practice, but there doesn't seem to be any other way to
  pass the dtb to U-Boot in the QEMU scenario. Using the -dtb argument
  does not work, I suppose because U-Boot doesn't use the same
  mechanics as the kernel when it's booting.

* Load addresses have been changed to fit QEMU use case.

People wanting to get a more detailed, yet somewhat isolated, diff
between this and the original, can run this command:

  git diff c6c26a05b89f25a06e7562f8c2071b60fd0c9eac~1 -- \
      $( git diff-tree --diff-filter=A -r --name-only HEAD~1 HEAD)

(Make sure to either check out this commit first, or replace HEAD with
the commit ID of this commit)

Signed-off-by: Kristian Amlie <kristian.amlie@northern.tech>