gpio: pca953x.c: add interrupt handling capability

Most of the GPIO expanders controlled by the pca953x driver are able to
report changes on the input pins through an *INT pin.

This patch implements the irq_chip functionality (edge detection only).

The driver has been tested on an Arcom Zeus.

[akpm@linux-foundation.org: the compiler does inlining for us nowadays]
Signed-off-by: Marc Zyngier <maz@misterjones.org>
Cc: Eric Miao <eric.y.miao@gmail.com>
Cc: Haojian Zhuang <haojian.zhuang@gmail.com>
Cc: David Brownell <david-b@pacbell.net>
Cc: Nate Case <ncase@xes-inc.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/i2c/pca953x.h

@@ -13,6 +13,9 @@ struct pca953x_platform_data {
 	/* initial polarity inversion setting */
 	uint16_t	invert;
 
+	/* interrupt base */
+	int		irq_base;
+
 	void		*context;	/* param to setup/teardown */
 
 	int		(*setup)(struct i2c_client *client,