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>