watchdog: omap: put struct watchdog_device into driver data

This way only a single allocation is needed (per device). Also this
simplifies the data structure used by the driver because there is no
need anymore to link from one struct to the other (by means of
watchdog_{set,get}_drvdata).

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