[WATCHDOG] mv64x60_wdt: Get register address from platform data

Previously, the address of the watchdog timer registers was
retrieved by calling a global function, mv64x60_get_bridge_vbase().
That function doesn't exist in arch/powerpc.  Instead, we now get
the register address from a platform data resource and ioremap
the registers within the driver.

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