gpio: sama5d2-piobu: Depend on OF_GPIO

This driver clearly needs OF_GPIO so depend on it.
Fixes a build error.

Cc: Andrei Stefanescu <Andrei.Stefanescu@microchip.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

drivers/gpio/Kconfig

@@ -441,6 +441,7 @@ config GPIO_REG
 config GPIO_SAMA5D2_PIOBU
 	tristate "SAMA5D2 PIOBU GPIO support"
 	depends on MFD_SYSCON
+	depends on OF_GPIO
 	select GPIO_SYSCON
 	help
 	  Say yes here to use the PIOBU pins as GPIOs.