Docs: Bump cmake version to 3.15

To align with current buildsystem. Remove references to special-cases
for some compilers.

Change-Id: I03447f747578922f398e77e800a46a8d4e732d0e
Signed-off-by: Raef Coles <raef.coles@arm.com>
diff --git a/docs/getting_started/tfm_sw_requirement.rst b/docs/getting_started/tfm_sw_requirement.rst
index 9070ef3..2f7b1c8 100644
--- a/docs/getting_started/tfm_sw_requirement.rst
+++ b/docs/getting_started/tfm_sw_requirement.rst
@@ -61,9 +61,7 @@
 CMake
 *****
 
-The build-system is CMake based and supports the versions 3.13 or higher.
-Please check `Exceptions and special cases`_ section if you need a higher
-version of CMake.
+The build-system is CMake based and supports the version 3.15 or higher.
 
     - Please use the latest build version available.
     - Recent versions of CMake can be downloaded from
@@ -348,12 +346,6 @@
       specific products and licenses as explained in
       `product-and-toolkit-configuration <https://developer.arm.com/products/software-development-tools/license-management/resources/product-and-toolkit-configuration>`__.
 
-    - ARMClang requires CMake version 3.15 or higher
-
-IAR toolchain
-=============
-    - IAR requires CMake version 3.14 or higher.
-
 MikTeX
 ======
     - When building the documentation the first time, MikTeX might prompt for