dt-bindings: display/msm: dsi-controller-main: Allow refgen-supply

DSI host needs REFGEN to be enabled (if it's present on a given platform).
Allow consuming it.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/545335/
Link: https://lore.kernel.org/r/20230628-topic-refgen-v3-3-9fbf0e605d23@linaro.org


Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>