Fix sparse warnings:

drivers/soc/ti/pm33xx.c:144:27: warning: symbol 'rtc_wake_src' was not declared. Should it be static?
drivers/soc/ti/pm33xx.c:160:5: warning: symbol 'am33xx_rtc_only_idle' was not declared. Should it be static?
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>