Add and document the reserved memory region property in the
mdss-common schema.

For now (sdm845-db845c), it points to a framebuffer memory
region reserved by the bootloader for splash screen.
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Amit Pundir <amit.pundir@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/549376/
Link: https://lore.kernel.org/r/20230726132719.2117369-1-amit.pundir@linaro.orgSigned-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>