arm64: dts: mediatek: Add initial MT7988A and BPI-R4

MT7988A (AKA MediaTek Filogic 880) is a quad-core ARM Cortex-A73
platform designed for Wi-Fi 7 devices (there is no wireless on SoC
though). The first public MT7988A device is Banana Pi BPI-R4.

Many SoC parts remain to be added (they need their own bindings or
depend on missing clocks). Those present block however are correct and
having base .dtsi will help testing & working on missing stuff.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Link: https://lore.kernel.org/r/20240108085228.4727-3-zajec5@gmail.com
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>
Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>