backlight: qcom-spmi-wled: Add support for QCOM SPMI WLED driver

qcom-spmi-wled driver adds support for controlling backlight for
LCD panels that uses QTI PMICs like pmi8998, PM8150L and PM6150L.

This snapshot is taken as of msm-4.14
commit 9bb584ae3a9d ("msm/sde/rotator: Add rev checks for sdmmagpie").

Change-Id: Ie2d3e1999cef12a83b301d9e838a7425e18ca9d7
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>