watchdog: imx2_wdt: set watchdog parent device

If on watchdog device registration a parent device is not set, then
the registered watchdog is considered to be a virtual device:

  /sys/devices/virtual/watchdog/watchdog0
  /sys/devices/virtual/watchdog/watchdog1

Setting a correct reference to a platform device allows to
distinguish multiple instances of iMX2+ hardware watchdogs:

  /sys/devices/soc0/soc/2000000.aips-bus/20bc000.wdog/watchdog/watchdog0
  /sys/devices/soc0/soc/2000000.aips-bus/20c0000.wdog/watchdog/watchdog1

Signed-off-by: Vladimir Zapolskiy <vladimir_zapolskiy@mentor.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>