Blackfin arch: Start untangling the CPLB handling code.

 - Move cache initialization to C from assembly.
 - Move anomaly workaround for writing [ID]MEM_CONTROL to assembly, so
   that we don't have to mess around with .align directives in C source.
 - Fix a bug where bfin_write_DMEM_CONTROL would write to IMEM_CONTROL
 - Break out CPLB related code from kernel/setup.c into their own file.
 - Don't define variables in header files, only declare them.

Signed-off-by: Bernd Schmidt <bernd.schmidt@analog.com>
Signed-off-by: Bryan Wu <bryan.wu@analog.com>