mtd: OneNAND: OMAP2/3: add support for command line partitioning

Add the ability to parse MTD partition information from the
kernel command line.

Note that a pointless BUG_ON is removed, as are redundant
calls to 'del_mtd_partitions()' and 'del_mtd_device()'
because they are also done by 'onenand_release()'.

Finally note that 'add_mtd_device()' returns 1 on failure
so the error condition was incorrect.

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