mfd: Add basic WM831x OTP support

The WM831x series of devices use OTP (One Time Programmable, a type
of PROM) to store system configuration. At run time this data is
visible via registers.

Currently the only explicitly supported feature is that the unique
ID provided by every WM831x device is exported to user space via
sysfs. Other configuration data may be read by system-specific
code in the pre_init() and post_init() platform data operations.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>