arm64: defconfig: enable MVPP2

The MVPP2 network driver is used for the ARM64 Marvell Armada 7K and 8K
platforms, so enable it in the arm64 defconfig.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>