Fix sparse warning:

drivers/rtc/rtc-opal.c:227:5:
 warning: symbol 'opal_tpo_alarm_irq_enable' was not declared. Should it be static?
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>