report changes on the input pins through an *INT pin.

This patch implements a very basic way for the platform code to be alerted
when some input changes (both edge detection).  It can be seen as a poor
man irq_chip.

Alternatively, this could be turned into a full-fledged irq_chip, though
I'm not sure it is worth the complexity.

The driver has been tested on an Arcom Zeus.
Signed-off-by: Marc Zyngier <maz@misterjones.org>
Cc: Eric Miao <eric.y.miao@gmail.com>
Cc: David Brownell <david-b@pacbell.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>