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-4.19 kernel
'commit 9b82b280d402 ("arm64: defconfig: Enable dm-snapshot")'.

Change-Id: Ib32b1185e50032db66a5db833c2a39f92eafdfb4