watchdog: docs: add an entry for imx2_wdt

Add an entry for imx2_wdt in watchdog-parameters.txt

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Wolfram Sang <w.sang@pengutronix.de>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>