fdt_support: fix an endian bug of fdt_initrd()

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_initrd() invoked write_cell(), which always swaps byte order.
It means the function only worked on little endian architectures.
(On big endian architectures, the byte order should be kept as it is)

This commit uses cpu_to_fdt32() and cpu_to_fdt64()
and deletes write_cell().

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