sp5100_tco: Add AMD Mullins platform support

AMD Mullins watchdog is fully compatible to the previous Hudson chipset,
reuse the existent sp5100_tco driver.

Signed-off-by: Denis Turischev <denis.turischev@compulab.co.il>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>