dt-bindings: mfd: qcom,spmi-pmic: Document flash LED controller

The Qualcomm SPMI PMIC come also with a LED controller for flash LEDs,
already used in DTS:

  sm8250-hdk.dtb: pmic@5: 'led-controller@d300' does not match any of the regexes: ...
  From schema: Documentation/devicetree/bindings/mfd/qcom,spmi-pmic.yaml

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20230416123203.299740-1-krzysztof.kozlowski@linaro.org