fdt: Add fdt_first/next_region() functions

These have been sent upstream but not accepted to libfdt. For now, bring
these into U-Boot to enable fdtgrep to operate. We will use fdtgrep to
cut device tree files down for SPL.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/lib/libfdt/Makefile b/lib/libfdt/Makefile
index 2f5413f..934d614 100644
--- a/lib/libfdt/Makefile
+++ b/lib/libfdt/Makefile
@@ -6,4 +6,4 @@
 #
 
 obj-y += fdt.o fdt_ro.o fdt_rw.o fdt_strerror.o fdt_sw.o fdt_wip.o \
-	fdt_empty_tree.o fdt_addresses.o
+	fdt_empty_tree.o fdt_addresses.o fdt_region.o