watchdog: qcom: document device tree bindings

The Qualcomm Krait Processor Sub-system (KPSS) contains one or more
instances of the WDT.  Provide documentation on how to describe these in
the device tree.

Signed-off-by: Josh Cartwright <joshc@codeaurora.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>