dt-bindings: mailbox: Add MT8196 GPUEB Mailbox

The MediaTek MT8196 SoC includes an embedded MCU referred to as "GPUEB",
acting as glue logic to control power and frequency of the Mali GPU.
This MCU runs special-purpose firmware for this use, and the main
application processor communicates with it through a mailbox.

Add a binding that describes this mailbox.

Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Nicolas Frattaroli <nicolas.frattaroli@collabora.com>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>