net: mt7628-eth: make phy link up detection optional via DT

net: mt7628-eth: make phy link up detection optional via DT

The mt7628 has an embedded ethernet switch (5 phy ports + 1 cpu port).
Although in IOT mode only port0 is usable, the phy0 is still connected
to the switch, not the ethernet gmac directly.

This patch rewrites it and makes it optional. It can be turned on by adding
mediatek,poll-link-phy = <?> explicitly into the eth node. By default the
driver is switch mode with all 5 phy ports working without link detection.

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