arm64: dts: mt7986: add port@5 as CPU port

On MediaTek MT7986b RFB and MediaTek MT7986a RFB, port 5 of the MT7531
switch is connected to the second MAC of the SoC as a CPU port. Add the
port and set up the second MAC on the bindings.

Signed-off-by: Arınç ÜNAL <arinc.unal@arinc9.com>
Link: https://lore.kernel.org/r/20240219-for-mediatek-v1-2-7078f23eab82@arinc9.com


Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>