Currently the ipq806x dwmac driver is almost always used attached to the
CPU port of a switch and phy-mode was always set to "rgmii" or "sgmii".

Some device came up with a special configuration where the PHY is
directly attached to the GMAC port and in those case phy-mode needs to
be set to "rgmii-id" to make the PHY correctly work and receive packets.

Since the driver supports only "rgmii" and "sgmii" mode, when "rgmii-id"
(or variants) mode is set, the mode is rejected and probe fails.

Add support also for these phy-modes to correctly setup PHYs that requires
delay applied to tx/rx.
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>