arm64: dts: r8a7796: Add Cortex-A53 CPU cores

This patch adds Cortex-A53 CPU cores of R8A7796 SoC, and sets a total of
6 cores (2 x Cortex-A57 + 4 x Cortex-A53).

Based on 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>