watchdog: mt7621-wdt: avoid static global declarations

Instead of using static global definitions in driver code, refactor code
introducing a new watchdog driver data structure and use it along the
code.

Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Link: https://lore.kernel.org/r/20230214103936.1061078-5-sergio.paracuellos@gmail.com


[groeck: unsigned -> unsigned int]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>