enable gmac in rk3288-evb-rk808.dts

changes since v2:
1. add fixed regulator for PHY
2. remove power-gpio, reset-gpio, phyirq-gpio, pmu_regulator setting
3. add "snps,reset-gpio", "snps,reset-active-low;" "snps,reset-delays-us"
Signed-off-by: NRoger Chen <roger.chen@rock-chips.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>