mvebu: neta: a37xx: Add fixed link support to neta driver

Add support for fixed link to NETA driver.
This feature requreed for proper support of SFP modules
and onboard connected devices like Ethernet switches

Signed-off-by: Konstantin Porotchkin <kostap@marvell.com>
Signed-off-by: Terry Zhou <bjzhou@marvell.com>
Cc: Stefan Roese <sr@denx.de>
Cc: Igal Liberman <igall@marvell.com>
Cc: Nadav Haklai <nadavh@marvell.com>
Signed-off-by: Stefan Roese <sr@denx.de>