dt-bindings: display/msm/dsi: allow specifying TE source

Command mode panels provide TE signal back to the DSI host to signal
that the frame display has completed and update of the image will not
cause tearing. Usually it is connected to the first GPIO with the
mdp_vsync function, which is the default. In such case the property can
be skipped.

Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Patchwork: https://patchwork.freedesktop.org/patch/598732/
Link: https://lore.kernel.org/r/20240613-dpu-handle-te-signal-v2-1-67a0116b5366@linaro.org