drivers: net: fsl_enetc: fix RGMII configuration

Add the missing RGMII PHY modes in which case the MAC has configure its
RGMII settings. The only difference between these modes is the RX and
TX delay configuration. A user might choose any RGMII mode in the device
tree.

Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Alex Marginean <alexm.osslist@gmail.com>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>