mtd: support ONFI multi lun NAND

With onfi a flash is organized into one or more logical units (LUNs).
A logical unit (LUN) is the minimum unit that can independently execute
commands and report status.

Mtd does not exploit LUN, so make it see a big single flash where size is
lun_size * number_of_lun.

Without this patch MT29F8G08ADBDAH4 size is 512MiB instead of 1GiB.

Artem: split long line on 2 shorter ones.

Signed-off-by: Matthieu Castet <matthieu.castet@parrot.com>
Acked-by: Florian Fainelli <ffainelli@freebox.fr>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>