Implement clock controller

[moojek]

Initial version - since I was lazy and it took very long to actually write this, I forgot exact requirements we made for this.
I'd like some input particularly on:

• in what units we want to operate the clock;
• do we want to support some kind of queuing or only one timer at a time;
• should it support custom callbacks or only one predefined;
• when requested should it run once or periodically.

[qbojj]

• Clock units should probably be what you are doing: same as time CSR (frequency from fdt /cpus/timer-frequency).
• We can prepare for queuing, If not within of the kernel itself, it will be used in userspace.
• I don't see any good reason for multiple callbacks - we are writing a microkernel. The only things that a timer interrupt would do is stop current task and switch to the process manager (the one that selects what is the next task to run and its time slice).
• As in the last point there is no real reason for cycling clock

[Kamilosok]

More docs

[Kamilosok]

Forgot one thing: add the docs pages at least for the api, or confirm that the full clock docs will be added in a separate pr. Thoug I'd prefer code to be shipped already documented.

[moojek]

Forgot one thing: add the docs pages at least for the api, or confirm that the full clock docs will be added in a separate pr. Thoug I'd prefer code to be shipped already documented.

I will add all the relevant docs within this pull request

[Kamilosok]

Docs done, LGTM