mtd: tests: add multiblock erase test to the mtd_speedtest

New multiblock erase speed test is added to mtd_speedtest.
It consists of 2-, 4-, 8-, 16-, 32- and 64-blocks at once
multiblock erase tests.

Signed-off-by: Roman Tereshonkov <roman.tereshonkov@nokia.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>