Add imx8qxp compatible string

Cc: Mark Rutland <mark.rutland@arm.com>
Cc: linux-serial@vger.kernel.org
Cc: devicetree@vger.kernel.org
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>