powerpc/85xx: Handle the lack of L2 cache on P2040/P2040E

The P2040/P2040E have no L2 cache.  So we utilize the SVR to determine
if we are one of these devices and skip the L2 init code in cpu_init.c
and release.  For the device tree we skip the updating of the L2 cache
properties but we still update the chain of caches so the CPC/L3 node
can be properly updated.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>