clocksource: devicetree: Document Conexant Digicolor timer binding

The Conexant CX92755 SoC provides 8 32-bit timers as part of its so called
"Agent Communication" block. Timers can be configures either as free running or
one shot. Each timer has a dedicated interrupt source in the CX92755 interrupts
controller. The first timer (Timer A) can also be configured as watchdog.

This commit adds devicetree binding definition of this hardware module. The
binding defined here should be reusable for other SoCs in the Digicolor series.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>