UBI: support ubi_num on mtd.ubi command line

I want to be able to add UBI volumes with specific numbers, but the
command line API doesn't have that atm.  Add an additional token to
support it.

Artem: amended the patch a little bit.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>