watchdog: omap_wdt: convert to new watchdog core

Convert omap_wdt to new watchdog core. On OMAP boards, there are usually
multiple watchdogs. Since the new watchdog core supports multiple
watchdogs, all watchdog drivers used on OMAP should be converted.

The legacy watchdog device node is still created, so this should not
break existing users.

Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Tested-by: Jarkko Nikula <jarkko.nikula@jollamobile.com>
Tested-by: Lokesh Vutla <lokeshvutla@ti.com>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>