cmake: Define new targets for AN552

Some of the platform-specific CMake dependencies are implicitly set by
the driver glue layer. The list of glue layer source files, which is
specific to the selected platform is also defined in the drivers dir.
This structure makes the project harder to read, and to scale (add new
platforms).

Move all the platform-specific CMake definitions to a platform-specific
directory.

Clean up the top level CMakeLists file for AN552 platform, moving most
of the details into separate subdirectories. The list of source files
that provide HAL implementation, as well as the list of CMake
dependencies, are kept top level.

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