mt7620: gsw: make IntPHY and ExtPHY share mdio addr 4 possible

mt7620: gsw: make IntPHY and ExtPHY share mdio addr 4 possible

To share mdio addr for IntPHY and ExtPHY,
as described in the documentation (MT7620_ProgrammingGuide.pdf).
(refer: http://download.villagetelco.org/hardware/MT7620/MT7620_ProgrammingGuide.pdf)

when port4 setup to work as gmac mode, dts like:

&gsw {
    mediatek,port4 = "gmac";
};

we should set SYSCFG1.GE2_MODE==0x0 (RGMII).
but SYSCFG1.GE2_MODE may have been set to 3(RJ-45) by uboot/default
so we need to re-set it to 0x0

before this changes:
gsw: 4FE + 2GE may not work correctly and MDIO addr 4 cannot be used by ExtPHY

after this changes:
gsw: 4FE + 2GE works and MDIO addr 4 can be used by ExtPHY

Signed-off-by: Chen Minqiang <ptpt52@gmail.com>