ARM: at91: time: move the system timer driver to drivers/clocksource

Import at91rm9200_time.c from mach-at91 as timer-atmel-st.c. Further cleanup is
required to get rid of the mach-at91 headers.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>