mfd: arizona: Integrate wm8997 into Arizona mfd

The wm8997 is a compact, high-performance audio hub CODEC with SLIMbus
interfacing, for smartphones, tablets and other portable audio devices
based on the Arizona platform.

This patch integrates the wm8997 into the Arizona mfd.

Signed-off-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>