ARM: mx28: add gpmi-nand support

add gpmi-nand device tree support.
add gpmi-nand pinctrl support.

Also enable the gpmi support for mx28-evk board.

Signed-off-by: Huang Shijie <b32955@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>