mips: dts: ralink: mt7621: change phy-mode of gmac1 to rgmii

Change phy-mode of gmac1 to rgmii on mt7621.dtsi. Same code path is
followed for delayed rgmii and rgmii phy-mode on mtk_eth_soc.c.

Signed-off-by: Arınç ÜNAL <arinc.unal@arinc9.com>
Reviewed-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>