mtd: gpmi: scan two nand chips

Some nand chip has two DIEs in a single chip, such as Micron MT29F32G08QAA.
Each die has its own chip select pin, so this chip acts as two nand
chips.

If we only scan one chip, we may find that we only get 2G for this chip,
but in actually, this chip's size is 4G.

So scan two chips by default.

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