watchdog: sp5100_tco: Convert to use watchdog subsystem

Convert to watchdog subsystem. As part of that rework, use devm functions
where possible, and replace almost all static variables with a dynamically
allocated data structure.

Cc: Zoltán Böszörményi <[email protected]>
Signed-off-by: Guenter Roeck <[email protected]>
Signed-off-by: Wim Van Sebroeck <[email protected]>

Author: groeck