net: mvneta: add support for fixed links

Following the introduction of of_phy_register_fixed_link(), this patch
introduces fixed link support in the mvneta driver, for Marvell Armada
370/XP SOCs.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: David S. Miller <davem@davemloft.net>