Blackfin arch: rewrite blackfin_invalidate_entire_dcache function

rewrite blackfin_invalidate_entire_dcache() in C for easier management,
better optimization, and so we take all SSYNC anomalies into account

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