ARM: shmobile: r8a7740 dtsi: Add L2 cache-controller node

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

The L2 cache is an ARM L2C-310 (r3p1-150rel0), of size 256 KiB (32 KiB x
8 ways).

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