ARM: at91: rm9200: switch back to clockevents_config_and_register

The timer code for at91rm9200 was already converted some time ago by
Shawn Guo in commit 838a2ae8 (ARM: use clockevents_config_and_register()
where possible) but because of a rounding issue in the timer core this
resulted in an easily reproducible oops. So it was reverted (commit
b7a8ca51 (ARM: at91: rm9200 fix time support)) which stopped the oops
from happening because min_delta_ns is increased by one in arch code
which stopped from problem from happening.

Now that the timer core problem is fixed (commit a4578ea (clockevents:
Sanitize ticks to nsec conversion)), we can switch back to the
clockevents_config_and_register helper.

Tested-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>