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_build_instruction_iar.rst b/docs/getting_started/tfm_build_instruction_iar.rst
index a320488..b9e8b2d 100644
--- a/docs/getting_started/tfm_build_instruction_iar.rst
+++ b/docs/getting_started/tfm_build_instruction_iar.rst
@@ -28,7 +28,7 @@
 .. code-block:: bash
 
     cd <TF-M base folder>
-    cmake -S . -B cmake_build -DTFM_PLATFORM=mps2/an521 -DTFM_TOOLCHAIN_FILE=toolchain_IARARM.cmake
+    cmake -S . -B cmake_build -DTFM_PLATFORM=arm/mps2/an521 -DTFM_TOOLCHAIN_FILE=toolchain_IARARM.cmake
     cmake --build cmake_build -- install
 
 Alternately using traditional cmake syntax
@@ -38,7 +38,7 @@
     cd <TF-M base folder>
     mkdir cmake_build
     cd cmake_build
-    cmake .. -DTFM_PLATFORM=mps2/an521 -DTFM_TOOLCHAIN_FILE=../toolchain_IARARM.cmake
+    cmake .. -DTFM_PLATFORM=arm/mps2/an521 -DTFM_TOOLCHAIN_FILE=../toolchain_IARARM.cmake
     make install
 
 Regression Tests for the AN521 target platform
@@ -47,7 +47,7 @@
 .. code-block:: bash
 
     cd <TF-M base folder>
-    cmake -S . -B cmake_build -DTFM_PLATFORM=mps2/an521 -DTFM_TOOLCHAIN_FILE=toolchain_IARARM.cmake -DTEST_S=ON -DTEST_NS=ON
+    cmake -S . -B cmake_build -DTFM_PLATFORM=arm/mps2/an521 -DTFM_TOOLCHAIN_FILE=toolchain_IARARM.cmake -DTEST_S=ON -DTEST_NS=ON
     cmake --build cmake_build -- install
 
 Alternately using traditional cmake syntax
@@ -57,7 +57,7 @@
     cd <TF-M base folder>
     mkdir cmake_build
     cd cmake_build
-    cmake .. -DTFM_PLATFORM=mps2/an521 -DTFM_TOOLCHAIN_FILE=../toolchain_IARARM.cmake -DTEST_S=ON -DTEST_NS=ON
+    cmake .. -DTFM_PLATFORM=arm/mps2/an521 -DTFM_TOOLCHAIN_FILE=../toolchain_IARARM.cmake -DTEST_S=ON -DTEST_NS=ON
     make install
 
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