clocksource: armada-370-xp: Replace WARN_ON with BUG_ON

If the clock fails to be obtained and the timer fails to be properly
registered, the kernel will freeze real soon. Instead, let's BUG()
where the actual problem is located.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Acked-by: Jason Cooper <jason@lakedaemon.net>
Acked-by: Gregory CLEMENT <gregory.clement@free-electrons.com>