arm/km: ethernet support for mgcoge3un

The phy is also configured with "RGMII clock transitions when data
stable" and "Class A driver for the direct backplane connection".

Signed-off-by: Valentin Longchamp <valentin.longchamp@keymile.com>
Signed-off-by: Holger Brunck <holger.brunck@keymile.com>
cc: Prafulla Wadaskar <prafulla@marvell.com>
cc: Heiko Schocher <hs@denx.de>