platform/x86: portwell-ec: Move watchdog device under correct platform hierarchy

platform/x86: portwell-ec: Move watchdog device under correct platform hierarchy

Without explicitly setting a parent for the watchdog device, the device is
registered with a NULL parent. This causes device_add() (called internally
by devm_watchdog_register_device()) to register the device under
/sys/devices/virtual, since no parent is provided. The result is:

DEVPATH=/devices/virtual/watchdog/watchdog0

To fix this, assign &pdev->dev as the parent of the watchdog device before
calling devm_watchdog_register_device(). This ensures the device is
associated with the Portwell EC platform device and placed correctly in
sysfs as:

DEVPATH=/devices/platform/portwell-ec/watchdog/watchdog0

This aligns the device hierarchy with expectations and avoids misplacement
under the virtual class.

Fixes: 835796753310 ("platform/x86: portwell-ec: Add GPIO and WDT driver for Portwell EC")
Signed-off-by: Ivan Hu <ivan.hu@canonical.com>
Link: https://lore.kernel.org/r/20250616074819.63547-1-ivan.hu@canonical.com
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>