of_gpio: Should use new <linux/gpio.h> header

Since commit 7560fa60fcdcdb0da662f6a9fad9064b554ef46c (gpio: <linux/gpio.h>
and "no GPIO support here" stubs) drivers can use GPIOs if they're available,
but don't require them.

This patch actually enables this feature.

Signed-off-by: Wolfgang Grandegger <[e-mail]>
Signed-off-by: Anton Vorontsov <[e-mail]>
Signed-off-by: Benjamin Herrenschmidt <[e-mail]>

include/linux/of_gpio.h

···
 #define __LINUX_OF_GPIO_H
 
 #include <linux/errno.h>
-#include <asm/gpio.h>
+#include <linux/gpio.h>
 
 #ifdef CONFIG_OF_GPIO