watchdog: max63xx: dynamically allocate device

This patch removes the static watchdog device for a new max63xx_wdt data
structure, and constifies the max63xx_timeout data.

The new structure contains pointers to pin access routines, which
abstracts mmap-specific code. This will ease future accesses like GPIO.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>