mtd-tests: flash_speed: Benchmark continuous reads

mtd-tests: flash_speed: Benchmark continuous reads

Currently the read throuput test tries:
- 1 page
- 2 pages
- 1 block (64 or more pages, usually)

But it might be interesting to see how the speed gradually increases,
eg. testing all number of pages from 1 to maybe 16, and then
arbitrarilly 32 and 64.

Let's add a -C parameter to enable this additional test.

The 2-page read/write tests are also moved under this new option.

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