powerpc/64s: support nospectre_v2 cmdline option

powerpc/64s: support nospectre_v2 cmdline option

BugLink: https://bugs.launchpad.net/bugs/1853142
commit d8f0e0b073e1ec52a05f0c2a56318b47387d2f10 upstream.

Add support for disabling the kernel implemented spectre v2 mitigation
(count cache flush on context switch) via the nospectre_v2 and
mitigations=off cmdline options.

Suggested-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Christopher M. Riedl <cmr@informatik.wtf>
Reviewed-by: Andrew Donnellan <ajd@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20190524024647.381-1-cmr@informatik.wtf
Signed-off-by: Daniel Axtens <dja@axtens.net>
CVE-2019-18660
Signed-off-by: Benjamin M Romer <benjamin.romer@canonical.com>
Acked-by: Sultan Alsawaf <sultan.alsawaf@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>