arm64: mvebu: Enable CONFIG_PHY_MARVELL in Armada7k/8k-DB defconfig

The Marvell PHY support is needed espescially for the A7040-DB with the
SGMII port (port 2). As without the marvell PHY driver configuration
for SGMII, ethernet won't work.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Stefan Chulski <stefanc@marvell.com>
Cc: Kostya Porotchkin <kostap@marvell.com>
Cc: Nadav Haklai <nadavh@marvell.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>