Blackfin arch: cleanup cache lock code

 - remove cheesy read_iloc() function
 - move invalidate_entire_icache function to lock.S
 - export proper prototypes for functions in lock.S
 - only build lock.S when BFIN_ICACHE_LOCK is enabled

Signed-off-by: Mike Frysinger <vapier.adi@gmail.com>
Signed-off-by: Bryan Wu <cooloney@kernel.org>