fdt_support: fix an endian bug of fdt_fixup_memory_banks

Data written to DTB must be converted to big endian order.
It is usually done by using cpu_to_fdt32(), cpu_to_fdt64(), etc.

fdt_fixup_memory_banks() invoked write_cell(), which always
swaps byte order.
It means the function only worked on little endian architectures.

This commit adds and uses a new helper function, fdt_pack_reg(),
which works on both big endian and little endian architrectures.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Acked-by: Simon Glass <sjg@chromium.org>