gpio: sunxi: Fix build with CONFIG_SPL_SERIAL=n

This driver uses simple_strtol(), so it needs SPL_STRTO. Before commit
88ca8e26958b6 ("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>
diff --git a/drivers/gpio/Kconfig b/drivers/gpio/Kconfig
index 149a62f..d7f37f0 100644
--- a/drivers/gpio/Kconfig
+++ b/drivers/gpio/Kconfig
@@ -365,6 +365,7 @@
 config SUNXI_GPIO
 	bool "Allwinner GPIO driver"
 	depends on ARCH_SUNXI
+	select SPL_STRTO if SPL
 	help
 	  Support the GPIO device in Allwinner SoCs.