[Blackfin] arch: remove duplicated prototypes for internal cplb structures from the global blackfin header

remove duplicated prototypes for internal cplb structures from
the global blackfin header as nothing else should be accessing these
Signed-off-by: NMike Frysinger <vapier.adi@gmail.com>
Signed-off-by: NBryan Wu <cooloney@kernel.org>