modules: cmsis, cmsis_6: only add the intended cmsis module

The current code base is meant to use cmsis for Cortex A and R and
cmsis_6 for Cortex M, but the build system is configured to include the
path for both when Cortex M is selected. This leaves us exposed to PR
using the old headers, that would not get caught in CI but would fail
the build on a project using Cortex M that only has the cmsis_6 module.

Change the cmsis module setting to only include the module files in the
intended case.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>