ARM: integrator: move AP timer to clocksource

This moves the timer/clocksource implementation for the
Integrator/AP down to drivers/clocksource and augments the
driver a little to use CLOCKSOURCE_OF_DECLARE(). Remove
the static mapping of the timer blocks while we're at it.

Tested on the Integrator/AP.

Acked-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>