gpio: mcp23s08: depend on OF_GPIO

The MCP drivers fails to compile on trial builds due to missing
Kconfig dependency on OF_GPIO. Fix it.

Cc: Lars Poeschel <poeschel@lemonage.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

drivers/gpio/Kconfig

@@ -717,7 +717,7 @@ config GPIO_MAX7301
 
 config GPIO_MCP23S08
 	tristate "Microchip MCP23xxx I/O expander"
-	depends on (SPI_MASTER && !I2C) || I2C
+	depends on (SPI_MASTER && !I2C) || I2C && OF_GPIO
 	help
 	  SPI/I2C driver for Microchip MCP23S08/MCP23S17/MCP23008/MCP23017
 	  I/O expanders.