pwm: sunxi: add support for PWM found on Allwinner A64

This commit adds basic support for PWM found on Allwinner A64.
It can be used for pwm_backlight driver (e.g. for Pinebook)

Signed-off-by: Vasily Khoruzhick <anarsoul@gmail.com>
diff --git a/arch/arm/include/asm/arch-sunxi/gpio.h b/arch/arm/include/asm/arch-sunxi/gpio.h
index 3334fb5..e4fe54d 100644
--- a/arch/arm/include/asm/arch-sunxi/gpio.h
+++ b/arch/arm/include/asm/arch-sunxi/gpio.h
@@ -172,6 +172,7 @@
 #define SUN8I_GPD_SDC1		3
 #define SUNXI_GPD_LCD0		2
 #define SUNXI_GPD_LVDS0		3
+#define SUNXI_GPD_PWM		2
 
 #define SUN5I_GPE_SDC2		3
 #define SUN8I_GPE_TWI2		3