libfdt: move fdt_find_regions() to fdt_region.c from fdt_wip.c

All the other fdt_*_region() functions are located in fdt_region.c,
while only fdt_find_regions() was added to fdt_wip.c, strangely.

Move it to the suitable place.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Simon Glass <sjg@chromium.org>