rtc: tps6594: Add driver for TPS6594 RTC

TPS6594 PMIC is a MFD. This patch adds support for
the RTC found inside TPS6594 family of PMIC.

Alarm is also supported.

Signed-off-by: Esteban Blanc <eblanc@baylibre.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Tested-by: Thomas Richard <thomas.richard@bootlin.com>
Link: https://lore.kernel.org/r/20231107094701.2223486-1-eblanc@baylibre.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>