gpio: pcf857x: enable gpio_to_irq() support

pcf857x chip has some pins, but interrupt pin is only 1 pin.
And gpiolib requests 1 interrupt per 1 gpio pin.

Thus, this patch added demuxed irq to pcf857x driver,
and enabled gpio_to_irq().

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
diff --git a/include/linux/i2c/pcf857x.h b/include/linux/i2c/pcf857x.h
index 0767a2a..781e6bd 100644
--- a/include/linux/i2c/pcf857x.h
+++ b/include/linux/i2c/pcf857x.h
@@ -10,6 +10,7 @@
  * @setup: optional callback issued once the GPIOs are valid
  * @teardown: optional callback issued before the GPIOs are invalidated
  * @context: optional parameter passed to setup() and teardown()
+ * @irq: optional interrupt number
  *
  * In addition to the I2C_BOARD_INFO() state appropriate to each chip,
  * the i2c_board_info used with the pcf875x driver must provide its
@@ -39,6 +40,8 @@
 					int gpio, unsigned ngpio,
 					void *context);
 	void		*context;
+
+	int		irq;
 };
 
 #endif /* __LINUX_PCF857X_H */