Platform: Adding Nuvoton M2351 Support

The NuMicro® M2351 microcontroller series is powered by Arm® Cortex®-M23
core with TrustZone® for Armv8-M architecture, which elevates the
traditional firmware security to the new level of robust software
security.

The porting will apply trusted-firmware-M to M2351 to make it easy to
create the security firmware on Nuvoton M2351 platform.

Signed-off-by: WSChang <wschang0@nuvoton.com>

Change-Id: I059618223fae8ab9ca085e9c998d51e3d12e8d86
diff --git a/platform/ext/index.rst b/platform/ext/index.rst
index d1ce4ef..34969b3 100644
--- a/platform/ext/index.rst
+++ b/platform/ext/index.rst
@@ -45,6 +45,13 @@
 
     target/nordic_nrf/**
 
+.. toctree::
+    :maxdepth: 4
+    :caption: Nuvoton
+    :glob:
+
+    target/nuvoton/**
+
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