arm64: dts: marvell: add XOR node for Armada 3700 SoC

Armada 3700 SoC comprise one dual-channel XOR engine and this
patch adds its according representation.

Signed-off-by: Marcin Wojtas <mw@semihalf.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>