dt-bindings: rtc: document the rtc-sh bindings

Add the binding documentation for rtc-sh which is an RTC for
SuperH and RZ/A SoCs.

Signed-off-by: Chris Brandt <chris.brandt@renesas.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>