ARM: dts: imx7: add CPU PMU support

Enable support for ARM Performance Monitoring Units available
on the Cortex-A7 CPU. There is only a single interrupt for the
PMU in both variants of the family, i.MX 7Solo and 7Dual.

Tested with perf on a i.MX 7Dual:
  hw perfevents: enabled with armv7_cortex_a7 PMU driver, 5 counters available

Signed-off-by: Stefan Agner <stefan@agner.ch>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>