ARM: dts: imx6qdl-sabresd: Add power key support

This patch adds support for imx6qdl-sabresd board's power
key, the key is named "SW1" on board, press it can wake up
system from suspend.

Add a new pinctrl entry for gpio keys and move all gpio
keys pin to this entry.

Signed-off-by: Anson Huang <b20788@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>