sysfs: Add attribute array to sysdev classes

Add a attribute array that is automatically registered and unregistered
to struct sysdev_class. This is similar to what struct class has.

A lot of drivers add list of attributes, so it's better to do
this easily in the common sysdev layer.

This adds a new field to struct sysdev_class. I audited the
whole tree and there are no dynamically allocated sysdev classes,
so this is fully compatible.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>