watchdog: imx2_wdt: Use register definition in regmap_write()

In order to improve readability it is better to pass the register name
definition rather than to pass its hardcoded offset.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>