dt-bindings: arm64: mediatek: Add MT7988A and BPI-R4

MT7988A is another MediaTek's SoC with just 1 device available right
now: Banana Pi BPI-R4.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20240108085228.4727-2-zajec5@gmail.com


Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>
Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>