rtc: rzn1: support input frequencies other than 32768Hz

rtc: rzn1: support input frequencies other than 32768Hz

When using the SCMP mode instead of SUBU, this RTC can also support
other input frequencies than 32768Hz. Also, upcoming SoCs will only
support SCMP.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/r/20250526095801.35781-8-wsa+renesas@sang-engineering.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>