libfdt: fdt_mem_rsv(): Fix comparison warnings

With -Wsign-compare, compilers warn about a mismatching signedness
in comparisons in fdt_mem_rsv().

Since all involved values must be positive, change the used types to be
unsigned.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Message-Id: <20200921165303.9115-3-andre.przywara@arm.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>