fdt_region: move fdt_region.c to common/ from lib/libfdt/

fdt_region: move fdt_region.c to common/ from lib/libfdt/

My goal is to sync lib/libfdt/ with scripts/dtc/libfdt/, that is,
make lib/libfdt/ contain only wrapper files.

fdt_region.c was written only for U-Boot to implement the verified
boot. So, this belongs to the same group as common/fdt_support.c,
which is a collection of U-Boot own fdt helpers.

Move lib/libfdt/fdt_region.c to common/fdt_region.c . This is
necessary only when CONFIG_(SPL_TPL_)_FIT_SIGNATURE is enabled.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>