mfd: Mark WM1811 GPIO6 register volatile for later revisions

For later chip revisions the WM1811 GPIO6 register is always volatile so
store the device revision when initialising the driver and then check at
runtime if we're running on a newer device.

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