clocksource: sun4i: Change CPU mask to cpu_possible_mask

The interrupt for the timer is a shared processor interrupt, so any CPU
found in the system can handle it. Switch to our cpumask to
cpu_possible_mask instead of cpumask_of(0).

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Reviewed-by: Stephen Boyd <sboyd@codeaurora.org>