if_awg: Respect rgmii-*id PHY configurations

phy-mode can be one of: rgmii, rgmii-id, rgmii-txid, rgmii-rxid; as this was
written, any of these alternate -id configurations would break as we fail to
configure syscon for rgmii. Instead, simply check that phy-mode is
configured for rgmii and we'll let the PHY driver handle any internal delay
configuration.

The pine64 should eventually specify phy-mode = "rgmii-txid" to address
gigabit issues when rx delay is configured, motivating this change.