mtd: nandsim: add support for 4KiB pages

I was going to play with a faulty nand image from real flash and noticed
that nandsim does not work with:
first_id_byte=0xec second_id_byte=0xd5 third_id_byte=0x51 fourth_id_byte=0xa6

This patch seems to fix it.

Signed-off-by: Sebastian Andrzej Siewior <sebastian@breakpoint.cc>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>