powerpc/mpc5200: fix bestcomm Kconfig dependencies

Without this patch it is possible to select drivers which require
bestcomm support without bestcomm support being selected.  This
patch reworks the bestcomm dependencies to ensure the correct
bestcomm tasks are always enabled.

Reported-by: Hans Lehmann <hans.lehmann@ritter-elektronik.de>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>