backlight: Add WM831x backlight driver

The WM831x series of PMICs provide DC-DC boost convertors and current
sinks which can be used together to drive LEDs for use as backlights.
Expose this functionality via the backlight API.

Since when used in this configuration the current sink and boost
convertor are very tightly coupled with a multi-stage startup for
the current sink which overlaps with the boost convertor startup
this driver bypasses the regulator API. Machine inititialisation
is responsible for ensuring that the regulators are not accessed
via both APIs.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Richard Purdie <rpurdie@linux.intel.com>