Input: prepare to sysfs integration

Add struct class_device to input_dev; add input_allocate_dev()
to dynamically allocate input devices; dynamically allocated
devices are automatically registered with sysfs.
Signed-off-by: NDmitry Torokhov <dtor@mail.ru>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>