rtc: digicolor: document device tree binding

Add a device tree binding documentation to the Real Time Clock hardware
block on the Conexant CX92755 SoC.  The CX92755 is from the Digicolor SoCs
series.  Other SoCs in that series may share the same hardware block.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>