mtd: allow MTD_NAND_GPMI_NAND to be built as module

Allow MTD_NAND_GPMI_NAND to be built as module.
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>

drivers/mtd/nand/Kconfig

@@ -452,7 +452,7 @@ config MTD_NAND_NANDSIM
 	  MTD nand layer.
 
 config MTD_NAND_GPMI_NAND
-        bool "GPMI NAND Flash Controller driver"
+        tristate "GPMI NAND Flash Controller driver"
         depends on MTD_NAND && MXS_DMA
         help
 	 Enables NAND Flash support for IMX23, IMX28 or IMX6.