rtc: sirfsoc: Make sysrtc_regmap_config static

Fix sparse warning:

drivers/rtc/rtc-sirfsoc.c:282:28: warning:
 symbol 'sysrtc_regmap_config' was not declared. Should it be static?

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>