mtd: gpmi: only scan two chips for imx6

We cannot scan two chips for imx23 and imx28:
  imx23: the Ready-Busy1 line is not connected for some board.
  imx28: we do not set the pinctrl for Ready-Busy1

So we only scan two chips for imx6.

Signed-off-by: Huang Shijie <b32955@freescale.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>