net: ftgmac100: support fixed link

Support fixed link in ftgmac100 driver. Fixed link is used on several
Meta OpenBMC platforms, such as Elbert (AST2620) and Wedge400 (AST2520).

Signed-off-by: Tao Ren <rentao.bupt@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>