devicetree: watchdog: add binding for Sigma Designs SMP8642 watchdog

This adds a binding for the watchdog in Sigma Designs SMP8642 and
similar devices.

Signed-off-by: Mans Rullgard <mans@mansr.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>