clocksource: em_sti: Set cpu_possible_mask to fix SMP broadcast

Update the STI driver by setting cpu_possible_mask to make EMEV2
SMP work as expected together with the ARM broadcast timer.

This breakage was introduced by:

f7db706b ARM: 7674/1: smp: Avoid dummy clockevent being preferred over real hardware clock-event

Without this fix SMP operation is broken on EMEV2 since no
broadcast timer interrupts trigger on the secondary CPU cores.

Signed-off-by: Magnus Damm <damm@opensource.se>
Tested-by: Simon Horman <horms+renesas@verge.net.au>
Reviewed-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>