x86/pkeys: Update documentation about availability

x86/pkeys: Update documentation about availability

Now that CPUs that implement Memory Protection Keys are publicly
available we can be a bit less oblique about where it is available.

Signed-off-by: Dave Hansen <[email protected]>
Acked-by: Thomas Gleixner <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Brian Gerst <[email protected]>
Cc: Denys Vlasenko <[email protected]>
Cc: H. Peter Anvin <[email protected]>
Cc: Josh Poimboeuf <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>