net: fec: use a more proper compatible string for i.MX6UL type device

i.MX6UL is a member in i.MX series family, the SOC FEC inherits from
i.MX6SX but removes some IP features, lets define a new type for fec
device.

Signed-off-by: Fugang Duan <fugang.duan@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>