arm64: dts: r8a7796: Add Cortex-A57 PMU node

Enable the performance monitor unit for the Cortex-A57 cores on the
R8A7796 SoC.

Signed-off-by: Takeshi Kihara <takeshi.kihara.df@renesas.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>