ARM: mvebu: Conform L2CC node with ePAPR specification by adding cache-level

For L2 cache controller node, cache-level property is mandatory. Let's
add it to Armada 370 and Armada XP device tree.

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