[MTD] support driver model updates

Follow-on patch to the previous driver model patch for the MTD
framework.  This one makes various MTD drivers connect to the
driver model tree, so /sys/devices/virtual/mtd/* nodes are no
longer present ... mostly drivers used on boards I have handy.

Based on a patch from Kay Sievers.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>