mtd: gpmi: add a new field for HW_GPMI_CTRL1

add the WRN_DLY_SEL field for HW_GPMI_CTRL1.
This field is used as delay for gpmi write strobe.

Signed-off-by: Huang Shijie <b32955@freescale.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>