ARM: mvebu: Enable Performance Monitor Unit on Armada XP/370 SoCs

The Armada 370 and XP SoCs have Cortex-A9 compatible CPUs, and with a
Performance Monitoring Unit.

Enable it so that we can have hardware-assisted perf support.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>