sysdev: Add utility functions for simple int/ulong variable sysdev attributes

This adds a new sysdev_ext_attribute that stores a pointer to the variable
it manages and some utility functions/macro to easily use them.

Previously all users wrote custom macros to generate show/store
functions for each variable, with this it is possible to avoid
that in many cases.

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