ARM: msm: add watchdog entries to DT timer binding doc

The watchdog has been reworked to use the same DT node as the timer.
This change is updating the device tree doc accordingly.

Signed-off-by: Mathieu Olivari <mathieu@codeaurora.org>
Acked-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>