mtd: gpmi: Remove "We support only one NAND chip" from bindings doc

This sentence "We support only one NAND chip now" is not true any more.
Multiple chips are supported. So lets remove this sentence to not
confuse anyone.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Huang Shijie <b32955@freescale.com>
Cc: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>