video: backlight: qcom-wled: Add PMI8994 compatible

Add a compatible for PMI8994 WLED. It uses the V4 of WLED IP.

Signed-off-by: Konrad Dybcio <konrad.dybcio@somainline.org>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>