arm64: dts: mt7622: add mt7531 dsa to mt7622-rfb1 board

Add mt7531 dsa to mt7622-rfb1 board for 5 giga Ethernet ports support.
mt7622 only supports 1 sgmii interface, so either gmac0 or gmac1 can be
configured as sgmii interface. In this patch, change to connect mt7622
gmac0 and mt7531 port6 through sgmii interface.

Signed-off-by: Landen Chao <landen.chao@mediatek.com>
Signed-off-by: David S. Miller <davem@davemloft.net>