Docs: Update required CMSIS_5 version to 5.5.0

This patch updates the documentation specifying that the
required version of CMSIS to be used is 5.5.0. It also
updates the define required during build time configuration
to DOMAIN_NS instead of __DOMAIN_NS.

Change-Id: Iebb157f981880c6fc89e60faf783499572757f4b
Signed-off-by: Antonio de Angelis <antonio.deangelis@arm.com>
diff --git a/docs/user_guides/tfm_build_instruction.rst b/docs/user_guides/tfm_build_instruction.rst
index b817353..f350251 100644
--- a/docs/user_guides/tfm_build_instruction.rst
+++ b/docs/user_guides/tfm_build_instruction.rst
@@ -60,7 +60,7 @@
 *******************
 External dependency
 *******************
-- CMSIS_5 is used to import RTX for the example non-secure app
+- `CMSIS_5` is used to import RTX for the example non-secure app
 - Mbed TLS and Mbed Crypto are used as crypto libraries on the secure side
 
 ****************
@@ -82,7 +82,12 @@
     git clone https://git.trustedfirmware.org/trusted-firmware-m.git
     git clone https://github.com/ARMmbed/mbedtls.git -b mbedtls-2.7.9
     git clone https://github.com/ARMmbed/mbed-crypto.git -b mbedcrypto-1.0.0
-    git clone https://github.com/ARM-software/CMSIS_5.git -b 5.2.0
+    git clone https://github.com/ARM-software/CMSIS_5.git -b 5.5.0
+
+.. 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.
 
 Build steps for the AN521 target platform:
 ==========================================