MIPS: Loongson64: dts: fix phy-related definition of LS7A GMAC

MIPS: Loongson64: dts: fix phy-related definition of LS7A GMAC

Currently the LS7A GMAC device tree node lacks a proper phy-handle
property pointing to the PHY node.

In addition, the phy-mode property specifies "rgmii" without any
internal delay information, which means the board trace needs to add 2ns
delay to the RGMII data lines; but that isn't known to happen on any
Loongson board. The ACPI-based initialization codepath, which is used on
LoongArch-based 3A5000 + 7A1000 hardwares, specifies "rgmii-id" phy
mode, which should be the one we are using.

Add the lacking phy-handle property and set proper phy-mode.

Tested on a LS3A4000_7A1000_NUC_BOARD_V2.1 board with YT8521S PHY.

Signed-off-by: Icenowy Zheng <zhengxingda@iscas.ac.cn>
Reviewed-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>