ARM: dts: Fix dm814x Ethernet by changing to use rgmii-id mode

Commit cd28d1d6 ("net: phy: at803x: Disable phy delay for RGMII mode")
caused a regression for dm814x boards where NFSroot would no longer work.

Let's fix the issue by configuring "rgmii-id" mode as internal delays are
needed that is no longer the case with "rgmii" mode.

Signed-off-by: Tony Lindgren <tony@atomide.com>