Blackfin: workaround anomaly 05000481 (corruption with ITEST MMRs)

Nothing actually needs to use these MMRs (as direct cache manipulation
is done with the DTEST MMRs), so simply hide the read funcs behind the
anomaly define.  They're generally unusable anyways when this anomaly
is in effect.

Signed-off-by: Robin Getz <robin.getz@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>