White LED(WLED) is a newly introduced feature from pmic 8922.
It can drive series/parallel combination of led strings that are
used to drive lcd backlight. Add support for WLED brightness
control in the existing pm8xxx led framework.
Change-Id: I991fdc500877beec01c2c7b5c822139d2aa960dc
Signed-off-by: Mohan Pallaka <mpallaka@codeaurora.org>