clocksource: pxa: Move PXA timer to clocksource framework

Move time.c from arch/arm/mach-pxa/time.c to
drivers/clocksource/pxa_timer.c.

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>