mfd: Core support for the WM8400 AudioPlus HiFi CODEC and PMU

The WM8400 is a highly integrated audio CODEC and power management unit
optimised for use in mobile multimedia applications.  This patch adds
core support for the WM8400 to the MFD subsystem.

Both I2C and SPI access are supported by the hardware but currently only
I2C access is implemented.  The code is structured to allow SPI support
to be slotted in later.

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>