powerpc/64s: Add barrier_nospec

[ Upstream commit a6b3964ad71a61bb7c61d80a60bea7d42187b2eb ]

A no-op form of ori (or immediate of 0 into r31 and the result stored
in r31) has been re-tasked as a speculation barrier. The instruction
only acts as a barrier on newer machines with appropriate firmware
support. On older CPUs it remains a harmless no-op.

Implement barrier_nospec using this instruction.

mpe: The semantics of the instruction are believed to be that it
prevents execution of subsequent instructions until preceding branches
have been fully resolved and are no longer executing speculatively.
There is no further documentation available at this time.

Signed-off-by: Michal Suchanek <msuchanek@suse.de>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>