ARM: dts: imx6sx-sdb: add gpio key support

This patch adds support for imx6sx-sdb board's gpio keys:

SW4(FUNC1): KEY_VOLUMEUP
SW5(FUNC2): KEY_VOLUMEDOWN

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