MIPS: dts: loongson: Fix GMAC phy node

phy-mode should be rgmii-id to match hardware configuration.

Also there should be a phy-handle to reference phy node.

Fixes: f8a11425

 ("MIPS: Loongson64: Add GMAC support for Loongson-2K1000")
Cc: stable@vger.kernel.org
Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>