net-next: mediatek: add fixed-phy support

The MT7623 SoC has a builtin gigabit switch. If we want to use it, GMAC1
needs to be configured using a fixed link speed and flow control settings.
The easiest way to do this is to used the fixed-phy driver, allowing us to
reuse the existing mdio polling code to setup the MAC.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: John Crispin <john@phrozen.org>
Signed-off-by: David S. Miller <davem@davemloft.net>