dt-bindings: rtc: Add Cirrus EP93xx

Add device tree bindings for the Cirrus Logic EP93xx RTC block
used in these SoCs.

Signed-off-by: Nikita Shubin <nikita.shubin@maquefel.me>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20230915-ep93xx-v4-13-a1d779dcec10@maquefel.me


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