libfdt: overlay: Fix comparison warning

With -Wsign-compare, compilers warn about a mismatching signedness in
a comparison in overlay_update_local_node_references().

This happens because the division of a signed int by an unsigned int
promotes the dividend to unsigned first (ANSI C standard 6.1.3.8).

As in this case we basically just divide by 4, we can do the division
separately earlier, which preserves the original type.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Message-Id: <20200921165303.9115-12-andre.przywara@arm.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>