libfdt: Fix signedness comparison warnings

This is a combination of upstream libfdt commits to fix warnings about
comparing signed and unsigned integers:
==========
scripts/dtc/libfdt/fdt.c: In function ‘fdt_offset_ptr’:
scripts/dtc/libfdt/fdt.c:137:18: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   if ((absoffset < offset)
...
==========

For a detailed description of the fixes, see the dtc repo:
https://git.kernel.org/pub/scm/utils/dtc/dtc.git/log/?id=73e0f143b73d808

For this patch the commits between 73e0f143b73d8088 and ca19c3db2bf62000
have been combined and adjusted for the slight differences in U-Boot's
libfdt code base.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
diff --git a/scripts/dtc/libfdt/fdt_wip.c b/scripts/dtc/libfdt/fdt_wip.c
index f64139e..c2d7566 100644
--- a/scripts/dtc/libfdt/fdt_wip.c
+++ b/scripts/dtc/libfdt/fdt_wip.c
@@ -23,7 +23,7 @@
 	if (!propval)
 		return proplen;
 
-	if (proplen < (len + idx))
+	if ((unsigned)proplen < (len + idx))
 		return -FDT_ERR_NOSPACE;
 
 	memcpy((char *)propval + idx, val, len);