allwinner: Introduce GPIO helper function

Many boards without a dedicated PMIC contain simple regulators, which
can be controlled via GPIO pins.

To later allow turning them off easily, introduce a simple function to
configure a given pin as a GPIO out pin and set it to the desired level.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
diff --git a/plat/allwinner/common/include/sunxi_private.h b/plat/allwinner/common/include/sunxi_private.h
index 9d20f60..54cc27e 100644
--- a/plat/allwinner/common/include/sunxi_private.h
+++ b/plat/allwinner/common/include/sunxi_private.h
@@ -18,5 +18,6 @@
 void sunxi_security_setup(void);
 
 uint16_t sunxi_read_soc_id(void);
+void sunxi_set_gpio_out(char port, int pin, bool level_high);
 
 #endif /* SUNXI_PRIVATE_H */