mtd: gpmi: add gpmi support for imx6sx

The gpmi's IP for imx6sx is nearly the same as the gpmi's IP for imx6q,
except the following two new features:

     (1) the new BCH contoller has 62-BIT correcting ECC strength
            (The BCH for imx6q only has 40-BIT ECC strength).

     (2) add the hardware Randomizer support.

This patch does the follow changes:

     (1) add a new macro GPMI_IS_MX6SX to represent the imx6sx's gpmi.

     (2) add a new macro GPMI_IS_MX6.
           We use this macro to initialize the same registers for both
         imx6sx and imx6q, and so on.

     (3) add a new gpmi_devdata instance, the gpmi_devdata_imx6sx, for
         imx6sx.

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