gpio: mcp23s08: switch to use gpiolib irqchip helpers

This switches the mcp23s08 driver to use the gpiolib irqchip
helpers.

Signed-off-by: Phil Reid <preid@electromag.com.au>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
diff --git a/drivers/gpio/Kconfig b/drivers/gpio/Kconfig
index 5f3429f..927be87 100644
--- a/drivers/gpio/Kconfig
+++ b/drivers/gpio/Kconfig
@@ -1091,6 +1091,7 @@
 
 config GPIO_MCP23S08
 	tristate "Microchip MCP23xxx I/O expander"
+	select GPIOLIB_IRQCHIP
 	help
 	  SPI/I2C driver for Microchip MCP23S08/MCP23S17/MCP23008/MCP23017
 	  I/O expanders.