Add device nodes for the L2 caches, and link the CPU nodes to them.

The L2 cache for the Cortex-A15 CPU cores is 2 MiB large (organized as
128 KiB x 16 ways).

The L2 cache for the Cortex-A7 CPU cores is 512 KiB large (organized as
64 KiB x 8 ways).
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>