ARM: shmobile: sh73a0 dtsi: Add L2 cache-controller node

Add the missing L2 cache-controller node, and link the CPU nodes to it.
This will allow migration to the generic l2c OF initialization.

The L2 cache is an ARM L2C-310 (r3p1), of size 512 KiB (64 KiB x 8
ways).

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>