ARM: dts: r8a7790: Add PMU device nodes

Enable support for the ARM Performance Monitor Units in the Cortex-A15
and Cortex-A7 CPU cores on R-Car H2 by adding device nodes for the two
PMUs.

New Linux output:

    hw perfevents: enabled with armv7_cortex_a15 PMU driver, 7 counters available
    hw perfevents: /pmu-1: failed to probe PMU!
    hw perfevents: /pmu-1: failed to register PMU devices!

The last two lines are due to the Cortex-A7 CPU cores being described in
DT, but not enabled by the firmware.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>