mtd: gpmi: initialize the timing registers only one time

The current code initializes the timing registers at very time
we call the gpmi_begin(). This really wastes the cpu cycles.

Add a new flag to let the gpmi driver initializes the timing registers
only one time.

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>