cpufreq: Use imx-cpufreq-dt for i.MX8MN's speed grading

Add i.MX8MN to blacklist, so that imx-cpufreq-dt driver can handle
speed grading bits just like other i.MX8M SoCs.
Signed-off-by: NAnson Huang <Anson.Huang@nxp.com>
Reviewed-by: NLeonard Crestez <leonard.crestez@nxp.com>
Signed-off-by: NViresh Kumar <viresh.kumar@linaro.org>

drivers/cpufreq/cpufreq-dt-platdev.c

@@ -109,6 +109,7 @@ static const struct of_device_id blacklist[] __initconst = {
 	{ .compatible = "fsl,imx7d", },
 	{ .compatible = "fsl,imx8mq", },
 	{ .compatible = "fsl,imx8mm", },
+	{ .compatible = "fsl,imx8mn", },
 
 	{ .compatible = "marvell,armadaxp", },