The PHY for MT7531/MT7988 are never actully turned ON/OFF for the
affected PHY as we are read/writing to the wrong PHY address.

This is caused by the fact that we use the MT753X_PHY_ADDR macro 2
times offsetting the address multiple times.

One in the _setup() function and one in the mt7531_mii_read/write.

Drop the additional usage of MT753X_PHY_ADDR in setup() to correctly
set the PHY.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>

mtk_eth.c contains not only the ethernet GMAC/DMA driver, but also
some ethernet switch initialization code. As we may add more switch
support in the future, it's better to move them out of mtk_eth.c to
avoid increasing the code complexity.

Since not all switches are supported for a particular board, Kconfig
options are added to allow user to select which switch should be
built into u-boot. If multiple switches are selected, auto-detecting
can also be enabled.

Signed-off-by: Weijie Gao <weijie.gao@mediatek.com>