dt-bindings: drm/bridge: MHDP8546 bridge binding changes for HDCP

Add binding changes for HDCP in the MHDP8546 DPI/DP bridge binding.

Signed-off-by: Parshuram Thombare <pthombar@cadence.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Robert Foss <robert.foss@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1618078508-30466-1-git-send-email-pthombar@cadence.com