imx8mn: support get_cpu_rev

Add a dummy cpu type and support get_cpu_rev for i.MX8MN

Signed-off-by: Peng Fan <peng.fan@nxp.com>
diff --git a/arch/arm/include/asm/mach-imx/sys_proto.h b/arch/arm/include/asm/mach-imx/sys_proto.h
index 1e627c8..459d73d 100644
--- a/arch/arm/include/asm/mach-imx/sys_proto.h
+++ b/arch/arm/include/asm/mach-imx/sys_proto.h
@@ -53,6 +53,8 @@
 #define is_imx8mmdl() (is_cpu_type(MXC_CPU_IMX8MMDL))
 #define is_imx8mms() (is_cpu_type(MXC_CPU_IMX8MMS))
 #define is_imx8mmsl() (is_cpu_type(MXC_CPU_IMX8MMSL))
+#define is_imx8mn() (is_cpu_type(MXC_CPU_IMX8MN))
+
 #define is_imx8qxp() (is_cpu_type(MXC_CPU_IMX8QXP))
 
 #ifdef CONFIG_MX6