net: sun8i-emac: Allow all RGMII PHY modes

net: sun8i-emac: Allow all RGMII PHY modes

So far all GBit users of the sun8i-emac driver were using the "rgmii"
PHY mode, even though this turns out to be wrong. It just worked because
the PHY driver doesn't do the proper setup (yet).
In fact for most boards the "rgmii-id" or "rgmii-txid" PHY modes are the
correct ones.

To allow the DTs to describe the phy-mode correctly, and to stay
compatible with Linux, at least allow those other RGMII modes in the
driver.

This avoids breakage if mainline DTs will be synced with U-Boot.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Acked-by: Heinrich Schuchardt <xypron.glpk@gmx.de>