MIPS: alchemy: cpu_all_mask is forbidden for clock event devices

change alchemy clock event device cpu_all_mask to cpu_possible_mask.
Gets rid of a warning, which then does the same substitution:
WARNING: CPU: 0 PID: 0 at kernel/time/clockevents.c:468 clockevents_register_device+0x130/0x140
rtcmatch2 cpumask == cpu_all_mask, using cpu_possible_mask instead

Signed-off-by: Manuel Lauss <manuel.lauss@gmail.com>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: Linux-MIPS <linux-mips@vger.kernel.org>