arm64: dts: r8a7796: Add Cortex-A57 CPU cores

This patch adds Cortex-A57 CPU cores to R8A7796 SoC for a total of
2 x Cortex-A57.

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