ARM: dts: imx7d-sdb: enable gpio buttons as wakeup source

This patch enables i.MX7D SDB board's below GPIO buttons
as wakeup sources:

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

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>