dt-bindings: rtc: rzn1: add optional second clock

The external crystal can be a second clock input. It is needed for the
SCMP counting method which allows using crystals different than 32768Hz.
It is also needed for an upcoming SoC which only supports the SCMP
method.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20250526095801.35781-6-wsa+renesas@sang-engineering.com


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