mtd: gpmi: remove unneccessary header

The whole gpmi-nand driver has turned to pure devicetree supported.
So the linux/mtd/gpmi-nand.h is not neccessary now. Just remove it,
and move some macros to the gpmi-nand driver itself.

Signed-off-by: Huang Shijie <shijie8@gmail.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>