rtc: mt6397: move some common definitions into rtc.h

move code to separate header-file to reuse definitions later
in poweroff-driver (drivers/power/reset/mt6323-poweroff.c)

Suggested-by: Frank Wunderlich <frank-w@public-files.de>
Signed-off-by: Josef Friedl <josef.friedl@speed.at>
Signed-off-by: Frank Wunderlich <frank-w@public-files.de>
Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>