watchdog: sama5d4: Cache MR instead of a partial config

.config is used to cache a part of WDT_MR at probe time and is not used
afterwards. Instead of doing that, actually cache MR and avoid reading it
every time it is modified.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>