ARM: dts: imx6q: remove the unused pins for gpmi-nand

The gpmi does not use the MX6Q_PAD_NANDF_CS2__NAND_CE2_B and
MX6Q_PAD_NANDF_CS3__NAND_CE3_B.

Just remove them.

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