ARM: at91: Add PMU support for sama5d3

ARM Performance Monitor Units are available on the sama5d3, add the support in
the dtsi.

Tested with perf and oprofile on the sama5d31ek.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>