docs/driver-model: Update device class docs

The driver model documentation was added to the kernel tree before
struct class was added to <linux/device.h>. Hence this patch that
updates the paragraph about struct class in
Documentation/driver-model/binding.txt.

Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Cc: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>