mfd: Add initialisation callback for WM8350

Some functions of the WM8350 require board-specific initialisation on
startup. Provide a callback to the WM8350 driver in platform data
for platforms to use to configure the chip.  Use of a callback allows
platforms to control the ordering of initialisation which can be
important.

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>