arm64: dts: renesas: r8a77980: add Cortex-A53 PMU support

arm64: dts: renesas: r8a77980: add Cortex-A53 PMU support

Describe the performance monitor unit (PMU) for the Cortex-A53 cores in
the R8A77980 SoC's device tree.

Based on the original (and large) patch by Vladimir Barinov.

Signed-off-by: Vladimir Barinov <vladimir.barinov@cogentembedded.com>
Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>