watchdog: sama5d4: Add comment explaining what happens on resume

Because suspending to RAM may lose the register values, they are restored
on resume. This is currently done unconditionally because there is
currently no way to know (from the driver) whether they have really been
lost or are still valid. Writing MR also pings the watchdog and this may
not be what is expected so add a comment explaining why it happens.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
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>