mtd: nand: gpmi: add gpmi dsm supend/resume support

mtd: nand: gpmi: add gpmi dsm supend/resume support

i.MX6SX supports deep sleep mode(DSM) that may turn off GPMI/BCH power
during suspend, add gpmi nand suspend/resume function to release DMA
channel in suspend function and re-init GPMI/BCH controller during
resume function.

Although it is not necessary to restore GPMI/BCH registers value for
i.MX6QDL, the code doesn't distinguish different platforms to keep the
code simple.

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