power_supply: Add driver for the PMU on WM831x PMICs

The WM831x PMICs provide power path management from three sources:
a wall supply, USB and a battery with integrated charger. They also
provide an additional backup supply with integrated for maintaining
always on functionality such as the RTC and monitoring of power
switches.

After some initial configuration at startup the device operates
autonomously, the driver simply provides reporting of the current
state.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>