arm64: dts: mediatek: Add initial MT7981B and Xiaomi AX3000T

arm64: dts: mediatek: Add initial MT7981B and Xiaomi AX3000T

MT7981B (AKA MediaTek Filogic 820) is a dual-core ARM Cortex-A53 SoC.
One of market devices using this SoC is Xiaomi AX3000T.

This is initial contribution with basic SoC support. More hardware block
will get added later. Some will need their bindings (like auxadc).

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://lore.kernel.org/r/20240111103928.721-3-zajec5@gmail.com
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>
Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>