net: lan743x: add fixed phy support for LAN7431 device

Add fixed_phy support at 1Gbps full duplex for the lan7431 device
if a phy not found over MDIO. Tested with a MAC to MAC connection
from LAN7431 to a KSZ9893 switch. This avoids the Driver open error
in LAN743x. TX delay and internal CLK125 generation is already
enabled in EEPROM.

Signed-off-by: Pavithra Sathyanarayanan <Pavithra.Sathyanarayanan@microchip.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>