dt-bindings: Add device tree binding for Goldfish RTC driver

Add documentation for DT binding of Goldfish RTC driver. The compatible
string used by OS for binding the driver is "google,goldfish-rtc".

Signed-off-by: Miodrag Dinic <miodrag.dinic@imgtec.com>
Signed-off-by: Goran Ferenc <goran.ferenc@imgtec.com>
Signed-off-by: Aleksandar Markovic <aleksandar.markovic@imgtec.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>