dt-bindings: display: panel: constrain 'reg' in DSI panels

DSI-attached devices could respond to more than one virtual channel
number, thus their bindings are supposed to constrain the 'reg' property
to match hardware.  Add missing 'reg' constrain for DSI-attached display
panels, based on DTS sources in Linux kernel (assume all devices take
only one channel number).

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20240509-dt-bindings-dsi-panel-reg-v1-3-8b2443705be0@linaro.org
Signed-off-by: Rob Herring (Arm) <robh@kernel.org>