LoongArch: Fix GMAC's phy-mode definitions in dts

The GMAC of Loongson chips cannot insert the correct 1.5-2ns delay. So
we need the PHY to insert internal delays for both transmit and receive
data lines from/to the PHY device. Fix this by changing the "phy-mode"
from "rgmii" to "rgmii-id" in dts.

Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>