gpio: mcp23s08: make driver depend on OF_GPIO

The MCP23S08 driver certainly accesses fields inside the
struct gpio_chip that are only available under CONFIG_OF_GPIO
not just CONFIG_OF, so update the Kconfig and driver to reflect
this.

Cc: Alexander Stein <alexander.stein@systec-electronic.com>
Cc: Phil Reid <preid@electromag.com.au>
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
diff --git a/drivers/gpio/Kconfig b/drivers/gpio/Kconfig
index 66a9410..24caedb 100644
--- a/drivers/gpio/Kconfig
+++ b/drivers/gpio/Kconfig
@@ -1131,6 +1131,7 @@
 
 config GPIO_MCP23S08
 	tristate "Microchip MCP23xxx I/O expander"
+	depends on OF_GPIO
 	select GPIOLIB_IRQCHIP
 	help
 	  SPI/I2C driver for Microchip MCP23S08/MCP23S17/MCP23008/MCP23017