gpio: sunxi: Fix build with CONFIG_SPL_SERIAL=n

This driver uses simple_strtol(), so it needs SPL_STRTO. Before commit
88ca8e26 ("disk: Add an option for partitions in SPL"), SPL_STRTO
was always selected indirectly. Now it is not, so select it here.

Signed-off-by: Samuel Holland <samuel@sholland.org>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>