dt-bindings: rtc: Add ST M48T86

Add YAML bindings for ST M48T86 / Dallas DS12887 RTC.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/lkml/61b9e036-7864-65c6-d43b-463fff896ddc@linaro.org/


Signed-off-by: Nikita Shubin <nikita.shubin@maquefel.me>
Link: https://lore.kernel.org/r/20230823-m48t86_device_tree-v2-1-21ff275f949d@maquefel.me


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