mfd: Add WM831x revision B support

Revision B of the WM831x devices changes the sense of the tristate
bit for GPIO configuration, inverting it to become an enable instead.
Take account of this in the gpiolib driver.

A current sink regulation status bit has also been added in revision B,
add a flag indicating if it's present but don't use it yet.

This revision also adds an interrupt on key up for the ON pin event
which the existing code is able to take advantage of.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>