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

SPI-attached devices could have more than one chip-select, thus their
bindings are supposed to constrain the 'reg' property to match hardware.
Add missing 'reg' constrain for SPI-attached display panels.

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-v1-2-8b2443705be0@linaro.org
Signed-off-by: Rob Herring (Arm) <robh@kernel.org>