cmake: Remove target-interface

`target-interface` is a platform-specific CMake target but it is
incorrectly located in the `drivers` directory, which hosts glue layer
for HAL.

For each of our platforms:
- Merge `target-interface` target into a platform-specific target.
- Copy platform-specific header files from drivers/target-interface/inc
  to platforms/x/device/inc.
- Since the header files do no try to be cross-platform any more, clean
  up the code a bit, and leave only the parts relevant for particular
platform.

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