cmake: Define new targets for AN555

- Remove unnecessary CMake dependencies for AN555 platform.
- Rename AN555 CMake targets for consistency, and to better communicate
  their purpose.
- Streamline the list of source files from the driver glue layer to
  contain only the ones relevant for the platform.
- Extract platform-specific components into separate subdirectories.

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