fdt_support: delete force argument of fdt_initrd()

After all, we have realized "force" argument is completely
useless. fdt_initrd() was always called with force = 1.

We should always want to do the same thing
(set appropriate value to the property)
even if the property already exists.

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