[POWERPC] CPM2: Make support for the CPM2 optional on 8560 based boards

Currently there is no way to disable the CPM2 support.  Some boards,
like the SBC8560 have their own external UART and don't have any direct
dependencies on the CPM for a serial console or anything else.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>