rtc: mt6397: improvements of rtc driver

- use regmap_read_poll_timeout to drop while-loop
- use devm-api to drop remove-callback

Suggested-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
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>