arm64: dts: r8a7796: Add Cortex-A53 PMU node

Enable the performance monitor unit for the Cortex-A53 cores on the
R8A7796 SoC.

Extracted from a patch by Takeshi Kihara in the BSP.

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