dt-bindings: backlight: qcom-wled: Add PMI8950 compatible

Document the compatible for the wled block found in PMI8950.

Signed-off-by: Luca Weiss <luca@z3ntu.xyz>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20221101161801.1058969-1-luca@z3ntu.xyz