[MTD] driver model updates (part 2)

1) Add more sysfs attributes: flags, size, erasesize, writesize,
   oobsize, numeraseregions, name

2) Move core_initcall() code into init_mtd().  The original approach
   does not work if CONFIG_MTD=m .

3) Add device_unregister() in del_mtd_device() so that devices get
   removed from sysfs as each driver is unloaded.

Signed-off-by: Kevin Cernekee <kpc.mtd@gmail.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>