dt-bindings: mediatek: add rdma-fifo-size description for mt8183 display

rdma fifo size may be different even in same SOC, add this
property to the corresponding rdma

Signed-off-by: Yongqiang Niu <yongqiang.niu@mediatek.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>