mtd: nandsim: Define CONFIG_NANDSIM_MAX_PARTS and use it instead of MAX_MTD_DEVICES

MAX_MTD_DEVICES is about to be removed.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>