gpio: sch311x: fix compilation error

Compilation failed on some platforms due to implicit inclusion of
<linux/io.h>, make this dependency explicit.
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

drivers/gpio/gpio-sch311x.c

@@ -18,6 +18,7 @@
 #include <linux/platform_device.h>
 #include <linux/gpio.h>
 #include <linux/bitops.h>
+#include <linux/io.h>
 
 #define DRV_NAME			"gpio-sch311x"