watchdog: add driver for Cortina Gemini watchdog

This add support for the Cortina systems Gemini (SL3516)
SoC watchdog.

I have tried to use all the right new kernel interfaces
and tested with busybox' "watchdog" command both to kick
and get timeouts and reboots.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>