mtd-tests: Read and write pages during speed tests

mtd-tests: Read and write pages during speed tests

The speed test does reads and writes of different sizes:
- eraseblock
- page
- two pages

At least this is the theory because, as opposed to the legacy kernel
module doing the same measurement, the userspace tool uses the subpage
size (hence accessing the same page 4, 8 or 16 times depending on the
subpage setting). Of course if the controller does not support subpages,
this issue is not visible.

Use mtd.min_io_size instead for non-NOR devices in order to get the
right bandwidth (at least one that fits the logs).

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: David Oberhollenzer <david.oberhollenzer@sigma-star.at>