imx: add i.MX8MN kconfig entry

Add i.MX8MN kconfig entry

Signed-off-by: Peng Fan <peng.fan@nxp.com>
diff --git a/arch/arm/mach-imx/imx8m/Kconfig b/arch/arm/mach-imx/imx8m/Kconfig
index f520075..b0932f1 100644
--- a/arch/arm/mach-imx/imx8m/Kconfig
+++ b/arch/arm/mach-imx/imx8m/Kconfig
@@ -12,6 +12,10 @@
 	bool
 	select IMX8M
 
+config IMX8MN
+	bool
+	select IMX8M
+
 config SYS_SOC
 	default "imx8m"