mtd: tests: add count parameter to mtd_speedtest

By default mtd_speedtest uses all the eraseblocks of the
MTD partition being tested.  For large partitions a
smaller number is sufficient and makes running the test
quicker.  For that reason, add a parameter 'count' to
specify the maximum number of eraseblocks to use for
testing.

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>