ARM: dts: imx7d-sdb: add gpio key support

Add support for imx7d-sdb board's gpio keys:

S1(FUNC1): KEY_VOLUMEUP
S3(FUNC2): KEY_VOLUMEDOWN

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Acked-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>