tests: drivers: pinctrl: add tests for API

Add a set of tests to check the API behavior. The API tests can only run
on a platform that does not have an actual pinctrl driver, e.g.
native_posix. The test itself implements a pinctrl mock driver and
provides the required "pinctrl_soc.h" header with required types/macros.
The implementation is used in the tests to verify the behavior of the
API or Devicetree macros.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>