mtd: gpmi: rename the functions from gpmi_nfc_* to gpmi_nand_*

The gpmi_nfc_* is the legacy name. In order to avoid the confusion,
The patch renames the gpmi_nfc_* functions to gpmi_nand_*.

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