leds: add Qualcomm PM8941 WLED driver

This adds support for the WLED ('White' LED) block on Qualcomm's
PM8941 PMICs.

(cooloney@gmail.com: remove unneeded semicolon)

Signed-off-by: Courtney Cavin <courtney.cavin@sonymobile.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@sonymobile.com>
Signed-off-by: Bryan Wu <cooloney@gmail.com>