mtd: expose subpage size via sysfs

MTD has got sysfs support in 2.6.30-rc1. But subpage size is not
exposed there - do this.

UBI utilities badly need this parameter. At the moment there is
no way to get subpage size - ioctls do not return it. And we
just got sysfs support, so we can easilly extend it with this
additional parameter.

This can be merged late in the development cycle because:
1. sysfs support has been just added - there are no users for
   it so far, even.
2. UBI utilities really need this parameter, and it is better
   not to delay this.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>