dt-bindings: rtc: add bindings for i.MX53 SRTC

Document the binding for i.MX53 SRTC implemented by rtc-mxc_v2

Signed-off-by: Patrick Bruenn <p.bruenn@beckhoff.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>