ARM: mvebu: use qsgmii phy-mode for Armada XP GP interfaces

The Armada XP GP isn't using rgmii-id connections between the MAC and
PHY, but instead a single QSGMII connection, which is a quad-SGMII
connection: a double pair of differential lines that are multiplexed
to convey the traffic of four network interfaces between a MAC and a
PHY.

Until now, the Armada XP GP was relying on the bootloader setting the
correct values in various configuration registers. With this change,
the mvneta driver can be used as a module on this platform.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Link: https://lkml.kernel.org/r/1397569821-5530-4-git-send-email-thomas.petazzoni@free-electrons.com
Tested-by: Arnaud Ebalard <arno@natisbad.org>
Tested-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>