ARM: dts: imx28: move extra gpmi-nand pins into board dts

Only pins GPMI_CE0N and GPMI_READY0 are generally used by other boards.
Move GPMI_CE1N and GPMI_READY1 into imx28-evk.dts to make gpmi_pins_a
more general, and can be used by other boards.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>