Blackfin: don't attempt to flush on-chip L1 SRAM regions

Since the on-chip L1 regions are not cacheable, there is no point in
trying to flush/invalidate them.  Plus, older Blackfin parts like to
trigger an exception (like BF533-0.3).

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