watchdog: omap: clearify device tree documentation

ti,hwmods doesn't belong into the compatible section but is a property
on it's own. Also reformat the section of required properties to match the
usual style of dt binding documents.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reviewed-by: Felipe Balbi <balbi@ti.com>
Acked-by: Felipe Balbi <balbi@ti.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>