clocksource: orion: Use atomic access for shared registers

Replace the driver-specific thread-safe shared register API
by the recently introduced atomic_io_clear_set().

Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Tested-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Tested-by: Willy Tarreau <w@1wt.eu>
Acked-by: Jason Cooper <jason@lakedaemon.net>
Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>