ARM: SMDK6410: Add support for PMIC LED on WM1192-EV1 PMIC module

The PMIC LED on the SMDK6410 CPU board is driven by GPIO4 of the
WM8312 PMIC. Provide software control of this LED.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Ben Dooks <ben-linux@fluff.org>