Documentation: mtd: amend the sysfs docs after mtdchar merge

The mtdchar module was merged with the mtdcore module, which means that the
MTD_CHAR Kconfig symbol does not exist any more.

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>