leds: add DT binding for Qualcomm PM8941 WLED block

This adds device tree binding documentation for the WLED ('White' LED)
block on Qualcomm's PM8941 PMICs.

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>