cppcheck: Remove old CMSIS folder from exclusion

Old `platform/ext/cmsis` was kept in, for tests to pass while the
refresh of CMSIS from version 5 to 6 was unmerged.
See in commit:
https://review.trustedfirmware.org/c/ci/tf-m-ci-scripts/+/26521

CMSIS was moved to libraries instead of being held in the source files,
as now it is compact.
Merged change in mention:
https://review.trustedfirmware.org/c/TF-M/trusted-firmware-m/+/26428

Signed-off-by: Gergely Korcsák <gergely.korcsak@arm.com>
Change-Id: Icaaa92b52e9285893945fb209e447a34088fe43a
1 file changed