arm64: dts: imx8mm: Add "fsl,imx8mq-src" as src's fallback compatible

i.MX8MM can reuse i.MX8MQ's src driver, add "fsl,imx8mq-src" as
src's fallback compatible to enable it.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>