rtc: pm8xxx: update kerneldoc for struct pm8xxx_rtc

rtc: pm8xxx: update kerneldoc for struct pm8xxx_rtc

The change from u8 ctrl_reg to const struct pm8xxx_rtc_regs *regs; did not
properly update the kerneldoc comment. Fixes:

drivers/rtc/rtc-pm8xxx.c:64: warning: Function parameter or member 'regs' not described in 'pm8xxx_rtc'

Fixes: c8d523a4b053 ("drivers/rtc/rtc-pm8xxx.c: rework to support pm8941 rtc")
Link: https://lore.kernel.org/r/20191122102212.400158-7-alexandre.belloni@bootlin.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>