cmake: Streamline CMSIS-Core targets

This HAL implementation repo requires only the CMSIS-Core API for
Cortex-M platforms.

- Remove CMSIS-5 git submodule.
- Use CMake's FetchContent to provide CMSIS-5 source code.
- Simplify the CMakeLists for CMSIS-Core; a single interface target
  is sufficient.

Signed-off-by: Filip Jagodzinski <filip.jagodzinski@arm.com>