Input: prepare to switching to struct device

In preparation to switching to struct device and class device
going away provide an alias to allow drivers that create devices
to use either input_dev->cdev.dev or input_dev->dev.parent to
put them into sysfs tree. The former will go away once conversion
to struct device is complete.

Signed-off-by: Dmitry Torokhov <dtor@mail.ru>