ARM: dts: r8a7794: Add PMU device node

Enable support for the ARM Performance Monitor Units in the Cortex-A7
CPU cores on R-Car E2 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>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>