arm64: dts: qcom: pmi8998: Add node for WLED

The PMI8998 PMIC has a WLED backlight controller, which is used on
most MSM8998 and SDM845 based devices: add a base configuration for
it and keep it disabled.

This contains only the PMIC specific configuration that does not
change across boards; parameters like number of strings, OVP and
current limits are product specific and shall be specified in the
product DT in order to achieve functionality.

Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@somainline.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lore.kernel.org/r/20210909123628.365968-1-angelogioacchino.delregno@somainline.org