leds: Add WM8350 LED driver

The voltage and current regulators on the WM8350 AudioPlus PMIC can be
used in concert to provide a power efficient LED driver.  This driver
implements support for this within the standard LED class.

Platform initialisation code should configure the LED hardware in the
init callback provided by the WM8350 core driver.  The callback should
use wm8350_isink_set_flash(), wm8350_dcdc25_set_mode() and
wm8350_dcdc_set_slot() to configure the operating parameters of the
regulators for their hardware and then then use wm8350_register_led() to
instantiate the LED driver.

This driver was originally written by Liam Girdwood, though it has been
extensively modified since then.

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