clock_control: API should allow callback to be specified

[masz-nordic]

When starting the clock asynchronously, there is no way to specify an additional callback function to propagate clock events to application.

API should include an optional registration of a handler that would be called when clock event (for example HF_CLOCK_STARTED) is triggered.

[cvinayak]

@carlescufi @pizi-nordic I agree on the requirement, but this needs API discussion and porting across various architectures and SoC from multiple vendors.

[pizi-nordic]

@cvinayak: That is true. The implementation must be generic. I will try to raise this topic on API meeting.

[carlescufi]

@pizi-nordic maybe you could propose an (optional) API that we could use, implementing it initially only on Nordic devices?

[pizi-nordic]

@carlescufi: OK.