Docs: Update docs on removing dependencies on CMSIS_5

As the dependencies on CMSIS RTX moving from CMSIS_5 to
tf-m-tests, update the documents accordingly.

Removed IAR specific instructions on replacing the RTX libraries.

Change-Id: I46befe3264c9d65251ebe427813b7956a1500e63
Signed-off-by: Kevin Peng <kevin.peng@arm.com>
Signed-off-by: TTornblom <thomas.tornblom@iar.com>
diff --git a/docs/getting_started/tfm_build_instruction.rst b/docs/getting_started/tfm_build_instruction.rst
index 121ef84..fd80b03 100644
--- a/docs/getting_started/tfm_build_instruction.rst
+++ b/docs/getting_started/tfm_build_instruction.rst
@@ -29,16 +29,7 @@
     cd <TF-M base folder>
     git clone https://git.trustedfirmware.org/TF-M/trusted-firmware-m.git
     git clone https://github.com/ARMmbed/mbed-crypto.git -b mbedcrypto-3.0.1
-    git clone https://github.com/ARM-software/CMSIS_5.git -b 5.5.0
-
-    # If CMSIS was cloned before the git-lfs dependency was installed,
-    # use the following command to download the binaries.
-    cd CMSIS_5 && git lfs pull
-
-.. Note::
-   `CMSIS_5` now uses `git-lfs` for storing large size binaries so the cloning
-   process has changed from previous releases. Please refer to the `CMSIS_5`
-   documentation for more details.
+    git clone https://git.trustedfirmware.org/TF-M/tf-m-tests.git
 
 .. Note::
    For building with Armclang compiler version 6.10.0, please follow the note