backlight: qcom-wled: Add support for WLED4 peripheral

WLED4 peripheral is present on some PMICs like pmi8998 and
pm660l. It has a different register map and configurations
are also different. Add support for it.

Signed-off-by: Kiran Gunda <kgunda@codeaurora.org>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>