mtd-tests: flash_speed: Clarify the number of pages in each set while measuring

mtd-tests: flash_speed: Clarify the number of pages in each set while measuring

So far speed calculations have only be done 1 page at a time or 2 pages
at a time in a block; so basically all the block was always read because
all blocks are multiple of 2. But in the future, if we want to extend
the number of pages in a single read, the final number of pages actually
read might be less than an erase block size, hence failing the throuput
calculations.

Make the number of pages in a set explicit.

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