spi doesn't need class_device

Make the SPI framework and drivers stop using class_device.  Update docs
accordingly ...  highlighting just which sysfs paths should be
"safe"/stable.

Signed-off-by: Tony Jones <tonyj@suse.de>
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Cc: Greg KH <greg@kroah.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>