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-4.19
commit b82b280d402 ("arm64: defconfig: Enable dm-snapshot").

Change-Id: I3eafddc1a2ce064552415516b168f452d0b3c638
Signed-off-by: Kiran Gunda <kgunda@codeaurora.org>