mfd: Add GPIO pin configuration support for WM8350

The WM8350 provides a number of user-configurable pins providing access
to various signals generated by the functions on the chip. These are
referred to as GPIO pins in the device documentation but in Linux terms
they are more general than that, providing configuration of alternate
functions.

This patch implements support for selecting the alternate functions for
these pins. They can also be used as GPIOs in the normal Linux sense -
a subsequent patch will add support for doing so.

This code was all written by Liam Girdwood and has had minor updates
and rearrangements 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>