Blackfin: allow cache funcs to be in L1 for IFLUSH Anomaly 05000491

Anomaly 05000491 says that IFLUSH cannot have certain types of memory
stalls triggered before it has completed in order to function correctly.
One such condition is that it be in L1 instruction.  So add a config
option to move it there, default it to on, and throw up a warning when
it is turned off and this anomaly exists.

Since the anomaly should be worked around, we can drop the older method
of calling IFLUSH multiple times.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>