nordic/peripherals interface - refactor to generic interfaces

[natersoz]

1. Consider using CMSIS peripheral interfaces when possible.
2. Move generic interface header files into a separate directory.
3. Timer & RTC interfaces contain Nordic specific instance information.

Example: The classes timer_observer_generic and timer_observable_generic are nearly generic in that they contain no external dependencies. Nope, false: They have the number 6 scattered about for the cc_index_limit which is Nordic specific.

This needs to be removed so that the interface is truly generic.