clocksource: sh_tmu: Set cpu_possible_mask to fix SMP broadcast

Update the TMU driver to use cpu_possible_mask as cpumask to make
r8a7779 SMP work as expected with or without the ARM TWD timer.

Signed-off-by: Magnus Damm <damm+renesas@opensource.se>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>