mfd: Add WM8350 subdevice registration helper

Most of the subdevices for the WM8350 code are registered in the same
fashion so factor out the code to do the initial registration.

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>