ARM: dts: r8a77470: Add PMU device node

Enable support for the ARM Performance Monitor Units in the Cortex-A7
CPU cores on RZ/G1C by adding a device node for the PMU.

New Linux output:

    hw perfevents: enabled with armv7_cortex_a7 PMU driver, 5 counters available

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Tested-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Reviewed-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>