net: bcmgenet: Add RGMII_RXID support

This adds the missing support for the PHY mode RGMII_RXID.
It's necessary for the Raspberry Pi 4.

Signed-off-by: Stefan Wahren <wahrenst@gmx.net>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>