The PL031 RTC binding has been in use for a long time, but never got
documented. Fortunately, it's simple and existing users are consistent.

Cc: Alessandro Zummo <a.zummo@towertech.it>
Cc: linux-rtc@vger.kernel.org
Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210607193718.3090202-1-robh@kernel.org/