watchdog: Migrate OMAP_WATCHDOG to Kconfig

Move this entry to Kconfig.  As it is a hardware watchdog, select
HW_WATCHDOG.  While we could default to enabling this for all platforms,
it is currently only enabled by default on AM33XX, so keep that logic
today.

Cc: Roger Meier <r.meier@siemens.com>
Signed-off-by: Tom Rini <trini@konsulko.com>