watchdog: omap_wdt: convert to devm_ functions

Use devm_kzalloc(), devm_request_mem_region() ande devm_ioremap()
to simplify the code.

Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>