powerpc/85xx: fix compatible property for the L2 cache node

The compatible property for the L2 cache node (on 85xx systems that don't
have a CPC) was using a value for the property length that did not match
the actual length of the property.

Signed-off-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>