dt-bindings: backlight: Add DT bindings for qcom-spmi-wled driver

Add DT bindings for Qualcomm SPMI WLED driver that supports
controlling backlight for LCD panels that uses pmi8998, PM8150L
and PM6150L.

This is taken as a snapshot from msm-4.14 kernel
'commit 9bb584ae ("msm/sde/rotator: Add rev checks for sdmmagpie")'

Change-Id: I31fa38cf6f45e4501c26b149be4020e9d0dc9814
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>