x86/mm/pkeys: Add missing Documentation

Stefan Richter noticed that the X86_INTEL_MEMORY_PROTECTION_KEYS option
in arch/x86/Kconfig references Documentation/x86/protection-keys.txt,
but the file does not exist.

This is a patch merging mishap: the final (v8) version of the pkeys
series did not include the documentation patch 32 and v7 included.
Add it now.

Reported-by: Stefan Richter <stefanr@s5r6.in-berlin.de>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Dave Hansen <dave@sr71.net>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-mm@kvack.org
Link: http://lkml.kernel.org/r/20151214190634.426BEE41@viggo.jf.intel.com
[ Added changelog. ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>

Signed-off-by: Ingo Molnar <mingo@kernel.org>