leds: Add WM831x status LED driver

The WM831x devices feature two software controlled status LEDs with
hardware assisted blinking.

The device can also autonomously control the LEDs based on a selection
of sources.  This can be configured at boot time using either platform
data or the chip OTP.  A sysfs file in the style of that for triggers
allowing the control source to be configured at run time.  Triggers
can't be used here since they can't depend on the implementation details
of a specific LED type.

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