eth: mtk-eth: add mt7531 switch support in mediatek eth driver

eth: mtk-eth: add mt7531 switch support in mediatek eth driver

mt7531 is a 7-ports switch with 5 embedded giga phys, and uses the same
MAC design of mt7530. The cpu port6 supports SGMII only. The cpu port5
supports RGMII or SGMII in different model.

mt7531 is connected to mt7622 via both RGMII and SGMII interfaces.
In this patch, mt7531 cpu port5 or port6 is configured to maximum
capability to align CPU MAC setting.

The dts has been committed in the commit 6efa450565cdc ("arm: dts:
mediatek: add ethernet and sgmii dts node for mt7622")

Signed-off-by: Landen Chao <landen.chao@mediatek.com>
Tested-by: Frank Wunderlich <frank-w@public-files.de>