arm64: dts: r8a7796: Add CA53 L2 cache-controller node

Add a device node for the Cortex-A53 L2 cache-controller.

The L2 cache for the Cortex-A53 CPU cores is 512 KiB large (organized as
32 KiB x 16 ways).

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>