From: Lars Poeschel <poeschel@lemonage.de>

This adds interrupt functionality for i2c chips to the driver.
They can act as a interrupt-controller and generate interrupts, if
the inputs change.
This is tested on a mcp23017 chip.

Signed-off-by: Lars Poeschel <poeschel@lemonage.de>
Device Tree and Open Firmware support activated because of IRQ functionality added to mcp23s08 driver