mtd-tests: flash_speed: Generalize read_eraseblock_by_2pages()

mtd-tests: flash_speed: Generalize read_eraseblock_by_2pages()

Right now there are only 2 pages that may be read continuously, but why
not trying more? At least when the continuous feature is out, this type
of benchmarking will be interesting. In order to facilitate later
additions, lets make this helper more generic and accept a global
'npages' variable as parameter (because this function is called in a
macro, it is simpler like that).

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