dt-binding: document the binding for mxc rtc

This adds documentation of device tree bindings for the
mxc rtc.

Cc: Rob Herring <robh+dt@kernel.org>
Cc: Pawel Moll <pawel.moll@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Ian Campbell <ijc+devicetree@hellion.org.uk>
Cc: Kumar Gala <galak@codeaurora.org>
Cc: devicetree@vger.kernel.org
Signed-off-by: Philippe Reynes <tremyfr@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>