Documentation: ABI: /sys/devices/system/cpu/ topology files

Add brief descriptions for the following sysfs files:

	/sys/devices/system/cpu/kernel_max
	/sys/devices/system/cpu/offline
	/sys/devices/system/cpu/online
	/sys/devices/system/cpu/possible
	/sys/devices/system/cpu/present

Excerpted the relevant information from Documentation/cputopology.txt
and pointed back to cputopology.txt as the authoritative source of
information.

Cc: Mike Travis <travis@sgi.com>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Alex Chiang <achiang@hp.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>