watchdog: honour hw_margin_ms DT property

Some watchdog devices, e.g. external gpio-triggered ones, must be
reset more often than once per second, which means that the current
rate-limiting logic in watchdog_reset() fails to keep the board alive.

gpio-wdt.txt in the linux source tree defines a "hw_margin_ms"
property used to specifiy the maximum time allowed between resetting
the device. Allow any watchdog device to specify such a property, and
then use a reset period of one quarter of that. We keep the current
default of resetting once every 1000ms.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Reviewed-by: Stefan Roese <sr@denx.de>