arm64: dts: mt7622: add mt7531 dsa to bananapi-bpi-r64 board

Add mt7531 dsa to bananapi-bpi-r64 board for 5 giga Ethernet ports support.

Signed-off-by: Landen Chao <landen.chao@mediatek.com>
Tested-By: Frank Wunderlich <frank-w@public-files.de>
Signed-off-by: David S. Miller <davem@davemloft.net>