mtd: add mtdpart option

As pointed by Jörg Krause one of the novelties of the 1.5.2 release is
the new mtdpart tool, so add an option for it.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>