[WATCHDOG] mv64x60_wdt: set up platform_device in platform code

The driver previously registered its platform device data in its own
init function--that's bogus.  Move that code to platform-specific
code in arch/ppc.  This is being done so that the platform code can
decide at runtime whether to initialize this driver or not.

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