clocksource/drivers: Add timer-of common init routine

The different drivers are all using the same pattern when initializing.

 1. Get the base address
 2. Get the irq number
 3. Get the clock
 4. Prepare and enable the clock
 5. Get the rate
 6. Request an interrupt

Instead of repeating again and again these steps in all the drivers, let's
provide a common init routine to give the opportunity to factor all of them
out.

We can expect a significant kernel size improvement when the common routine
will be used in all the drivers.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>