ARM: dts: socfpga: Add support for PMU

The dual Cortex-A9 MPCore inside socfpga has a standard PMU unit for
each core mapped in the DAP memory space. Add support for it!

Tested with perf on a Cyclone 5 SoC DK.

Reported-by: Alberto Dassatti <alberto.dassatti@heig-vd.ch>
Signed-off-by: Florian Vaussard <florian.vaussard@heig-vd.ch>
Tested-by: Alberto Dassatti <alberto.dassatti@heig-vd.ch>
Signed-off-by: Dinh Nguyen <dinguyen@kernel.org>