mtd: delete SBC82xx/SBC8560 MTD mapping support

The SBC8260 support was dropped back when we moved from ppc to
powerpc.  We are now also dropping the support for the EOL SBC8560,
so we can also delete this mapping support, as they were the only
users of it.

Artem: also remove the symbol from the Makefile.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>