boston: Setup memory ranges in FDT provided to Linux

The boston memory map isn't suited to the simple "all memory starting
from 0" approach that the MIPS arch_fixup_fdt() implementation takes.
Instead we need to indicate the first 256MiB of DDR from 0 and the rest
from 0x90000000. Implement ft_board_setup to do that.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>