[JUST RFC] add hardware independent clock source

[Antony Pavlov]

Then porting barebox to a new SoC/board it is possible
a situation then clocksource is broken or not yet present.
In the broken/absent clocksource situation serial barebox
console can't work correctly. Working on barebox for a DIGIC
chip I have faced this situation. To overcome the problem
I introduced the software-only clocksource.

The sample of hardware independent clock source is
in the following patch:

[JUST RFC] ARM: DIGIC: add csrc-dummy

Please comment on this approach!
May be there is some ready receipt for broken clocksource case?