mtd: Blackfin NFC: localize MMR bit masks

Convert all magic numbers into appropriate defines, and move the defines
out of the global namespace and into this one driver.  No other driver
needs to care about the MMR layout anyways.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>