net: mediatek: split ethernet switch code from mtk_eth.c

net: mediatek: split ethernet switch code from mtk_eth.c

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>