arm64: dts: r8a7795: pmu: switch to Cortex specific device nodes

Instead of using the generic armv8-pmuv3 compatibility use the more
specific Cortex A57 compatibility.

Signed-off-by: Dirk Behme <dirk.behme@gmail.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>