doc: board: Add Calxeda Highbank/Midway documentation

The Calxeda servers are using U-Boot as the primary bootloader, which
was shipped as part of a firmware upgrade package.
Even though the machines are considered legacy at this point, the port
still works, so deserves some documentation.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Simon Glass <sjg@chromium.org>