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/technical_references/profiles/tfm_profile_medium.rst b/docs/technical_references/profiles/tfm_profile_medium.rst
index ab552f1..ae24b98 100644
--- a/docs/technical_references/profiles/tfm_profile_medium.rst
+++ b/docs/technical_references/profiles/tfm_profile_medium.rst
@@ -430,7 +430,7 @@
 
    cd <TFM root dir>
    mkdir build && cd build
-   cmake -DTFM_PLATFORM=mps2/an521 \
+   cmake -DTFM_PLATFORM=arm/mps2/an521 \
          -DTFM_TOOLCHAIN_FILE=../toolchain_ARMCLANG.cmake \
          -DTFM_PROFILE=profile_medium \
          -DCMAKE_BUILD_TYPE=MinSizeRel \
@@ -444,7 +444,7 @@
 
    cd <TFM root dir>
    mkdir build && cd build
-   cmake -DTFM_PLATFORM=mps2/an521 \
+   cmake -DTFM_PLATFORM=arm/mps2/an521 \
          -DTFM_TOOLCHAIN_FILE=../toolchain_ARMCLANG.cmake \
          -DTFM_PROFILE=profile_medium \
          -DCMAKE_BUILD_TYPE=MinSizeRel \