ARM: dts: imx6qdl-sabreauto: add gpio keys support

Add i.MX6QDL SabreAuto board's gpio keys support, there
are 5 gpio keys on base board:

SW3: KEY_HOME;
SW4: KEY_BACK;
SW5: KEY_PROGRAM;
SW6: KEY_VOLUMEUP;
SW7: KEY_VOLUMEDOWN;

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>