led: leds-qpnp-wled: add support for qpnp wled

Add driver for wled (white led) module which is
part of PMIC. Wled supports backlight for the
devices. It supports four strings of leds.

Change-Id: I42bd1d7d7fe01c950f8c3ba6137a74485a185ae6
Signed-off-by: Mohan Pallaka <mpallaka@codeaurora.org>