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

S1(FUNC1): KEY_VOLUMEUP
S3(FUNC2): KEY_VOLUMEDOWN
Signed-off-by: NAnson Huang <Anson.Huang@nxp.com>
Acked-by: NDong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: NShawn Guo <shawnguo@kernel.org>