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>
diff --git a/common/Makefile b/common/Makefile
index 3471c47..2e7a090 100644
--- a/common/Makefile
+++ b/common/Makefile
@@ -110,6 +110,7 @@
 obj-$(CONFIG_ANDROID_AB) += android_ab.o
 obj-$(CONFIG_ANDROID_BOOT_IMAGE) += image-android.o image-android-dt.o
 obj-$(CONFIG_$(SPL_TPL_)OF_LIBFDT) += image-fdt.o
+obj-$(CONFIG_$(SPL_TPL_)FIT_SIGNATURE) += fdt_region.o
 obj-$(CONFIG_$(SPL_TPL_)FIT) += image-fit.o
 obj-$(CONFIG_$(SPL_)MULTI_DTB_FIT) += boot_fit.o common_fit.o
 obj-$(CONFIG_$(SPL_TPL_)IMAGE_SIGN_INFO) += image-sig.o