dt-bindings: rtc: Add TI K3 RTC description

This adds the documentation for the devicetree bindings of the Texas
Instruments RTC modules on K3 family of SoCs such as AM62x SoCs or
newer.

Signed-off-by: Nishanth Menon <nm@ti.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20220623170808.20998-2-nm@ti.com