ARM: dts: imx6q{dl}: add DTE pads for uart

The uart2 in the imx6q-arm2 board is used as a DTE uart,
this patch adds the necessary DTE pads for uart2.

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