mtd: fix compile error for gpmi-nand

The driver gpmi-nand should compile at least.  This patch adds the
missing gpmi-nand.h to fix the compile error below.

  CC      drivers/mtd/nand/gpmi-nand/gpmi-nand.o
  CC      drivers/mtd/nand/gpmi-nand/gpmi-lib.o
drivers/mtd/nand/gpmi-nand/gpmi-nand.c:25:33: fatal error: linux/mtd/gpmi-nand.h: No such file or directory
drivers/mtd/nand/gpmi-nand/gpmi-lib.c:21:33: fatal error: linux/mtd/gpmi-nand.h: No such file or directory

This header is grabbed from patch below, which has not been postponed
for merging.

  [PATCH v8 1/4] ARM: mxs: add GPMI-NAND support for imx23/imx28
  http://permalink.gmane.org/gmane.linux.drivers.mtd/37338



Signed-off-by: Huang Shijie <b32955@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Vinod Koul <vinod.koul@linux.intel.com>