[WATCHDOG] omap_wdt.c: another ioremap() fix

convert to use ioremap() and __raw_{read/write} friends.

Signed-off-by: Felipe Balbi <felipe.balbi@nokia.com>
Signed-off-by: George G. Davis <gdavis@mvista.com>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>