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: NSebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Tested-by: NWilly Tarreau <w@1wt.eu>
Acked-by: NJason Cooper <jason@lakedaemon.net>
Signed-off-by: NEzequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: NDaniel Lezcano <daniel.lezcano@linaro.org>