watchdog: ftwdt010: Make interrupt optional

The Moxart does not appear to be using the interrupt from the
watchdog timer, maybe it's not even routed, so as to support
more architectures with this driver, make the interrupt
optional.

While we are at it: actually enable the use of the interrupt
if present by setting the right bit in the control register
and define the missing control register bits.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>