dt-bindings: rtc: Add #clock-cells property

dt-bindings: rtc: Add #clock-cells property

The RTC in the JZ4770 is compatible with the JZ4760, but has an extra
register that permits to configure the behaviour of the CLK32K pin. The
same goes for the RTC in the JZ4780.

With this change, the RTC node is now also a clock provider on these
SoCs, so a #clock-cells property is added.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20230129120442.22858-2-paul@crapouillou.net
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>