plat: imx8mn: Add imx8mn basic support

Add imx8mn basic support

Signed-off-by: Jacky Bai <ping.bai@nxp.com>
Change-Id: Ibdfcc87700bfaf980e429f3a5fa08515218ae78d
diff --git a/docs/plat/imx8m.rst b/docs/plat/imx8m.rst
index 8acd13c..ba087a2 100644
--- a/docs/plat/imx8m.rst
+++ b/docs/plat/imx8m.rst
@@ -32,6 +32,7 @@
 
    Target_SoC should be "imx8mq" for i.MX8MQ SoC.
    Target_SoC should be "imx8mm" for i.MX8MM SoC.
+   Target_SoC should be "imx8mn" for i.MX8MN SoC.
 
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