MFD: ucb1x00-assabet: add support for UCB1x00 GPIO switches

Add support for UCB1x00 GPIO buttons found on the Assabet platform.
We can now trivially support these buttons as we have standardized
gpiolib, genirq and gpio keyboard support in place for this device.

Acked-by: Jochen Friedrich <jochen@scram.de>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>