Platform: Add arm folder for arm platforms

Put arm platforms into arm folder and remove some unused
cryptocell-312-runtime. It will be convenient for users and
developers.

Signed-off-by: Summer Qin <summer.qin@arm.com>
Change-Id: I1187a8f2b18903e531ce28b2d23be251ec1e2b45
diff --git a/docs/getting_started/tfm_user_guide.rst b/docs/getting_started/tfm_user_guide.rst
index d2b0193..bd9ce91 100644
--- a/docs/getting_started/tfm_user_guide.rst
+++ b/docs/getting_started/tfm_user_guide.rst
@@ -224,7 +224,7 @@
 .. Note::
 
     Before executing any images on Musca-B1 board, please check the
-    :doc:`target platform readme </platform/ext/target/musca_b1/sse_200/readme>`
+    :doc:`target platform readme </platform/ext/target/arm/musca_b1/sse_200/readme>`
     to have the correct setup.
 
 Example application with BL2 bootloader
@@ -416,7 +416,7 @@
 ============================================================================
 
 Follow the above procedures, but to create a unified hex please check the
-:doc:`Musca-B1 Secure Enclave readme </platform/ext/target/musca_b1/secure_enclave/readme>`.
+:doc:`Musca-B1 Secure Enclave readme </platform/ext/target/arm/musca_b1/secure_enclave/readme>`.
 
 ********************************************************
 Execute TF-M example and regression tests on MPS3 boards