dt-bindings: backlight: qcom-wled: Convert the wled bindings to .yaml format

Convert the qcom-wled bindings from .txt to .yaml format.
Also replace PM8941 to WLED3 and PMI8998 to WLED4.

Signed-off-by: Kiran Gunda <kgunda@codeaurora.org>
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>
Acked-by: Daniel Thompson <daniel.thompson@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>