dt-bindings: gpu: mali-bifrost: Add compatible for MT8188 SoC

Add a compatible for the MediaTek MT8188 SoC, with an integrated
ARM Mali G57 MC3 (Valhall-JM) GPU.

Acked-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Chen-Yu Tsai <wenst@chromium.org>
Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240611085602.491324-2-angelogioacchino.delregno@collabora.com