[WATCHDOG] mv64x60_wdt: Support the WDIOF_MAGICCLOSE feature

Disallow disabling of the watchdog timer unless a particular
character ('V') was recently written to the watchdog device.

Signed-off-by: Dale Farnsworth <dale@farnsworth.org>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>