mtd: introduce mtd_device_(un)register()

To prepare for the removal of add_mtd_device and add_mtd_partitions(),
introduce mtd_device_register().  This will create partitions if they
are supplied or register the whole device if there are no partitions.

Once all drivers are converted to use mtd_device_register(),
add_mtd_device() and add_mtd_partitions() will be made internal only.

v2: move kerneldoc to implementation file and fixup some kerneldoc
warnings.

Artem: tweak comments: remove junk tabs, use dots consistently.

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