rtc: rzn1: Add new RTC driver

Add a basic RTC driver for the RZ/N1.

Signed-off-by: Michel Pollet <michel.pollet@bp.renesas.com>
Co-developed-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20220516082504.33913-3-miquel.raynal@bootlin.com