Input: sirfsoc-onkey - set the capability of reporting KEY_POWER

commit a1a75210 moved to report EV_KEY event(KEY_POWER) instead of
reporting EV_PWR event(KEY_SUSPEND), but it didn't enable the capability, so
the KEY_POWER will not be reported to userspace by input core. this patch fixes
the issue.

Signed-off-by: Xianglong Du <Xianglong.Du@csr.com>
Signed-off-by: Barry Song <Baohua.Song@csr.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>