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.