mfd: Add I2C control support for WM8350

Implement the I2C control interface for the WM8350. This code was
originally written by Liam Girdwood and has been updated for submission
by Mark Brown.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Samuel Ortiz <sameo@openedhand.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>