arm64: dts: imx8qm: add compatible string for usdhc3

add compatible string for usdhc3

Cc: Rob Herring <robh+dt@kernel.org>
Cc: Sascha Hauer <kernel@pengutronix.de>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: devicetree@vger.kernel.org
Cc: Fabio Estevam <festevam@gmail.com>
Cc: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Oliver Graute <oliver.graute@kococonnector.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>