ARM: mvebu: Enable Performance Monitor Unit on Armada 375 SoC

The Armada 375 SoC has a Cortex-A9 CPU, and so the PMU is available
to be used. This commit enables it in the devicetree.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
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>