fec: convert to gpio descriptor

The driver can be trivially converted, as it only triggers the gpio
pin briefly to do a reset, and it already only supports DT.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>