arm64: defconfig: enable mv-xor driver

The mv-xor DMA driver is used for the XOR engine found in the ARM64
Marvell Armada 3720 SoC, so it makes sense to have it enabled in the
arm64 defconfig. A recent boot-time regression was found in mv-xor,
which would have been more easily noticed with this driver enabled by
default.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Robin Murphy <robin.murphy@arm.com>
Cc: John David Anglin <dave.anglin@bell.net>
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>