x86: Put 'nopat' in kernel-parameters

Add missing 'nopat' boot option into
Documentation/kernel-parameters.txt.

Signed-off-by: Jiri Kosina <jkosina@suse.cz>
LKML-Reference: <alpine.LNX.2.00.1001181703560.30977@pobox.suse.cz>
Signed-off-by: Ingo Molnar <mingo@elte.hu>