powerpc/fsl: Add nospectre_v2 command line argument

powerpc/fsl: Add nospectre_v2 command line argument

When the command line argument is present, the Spectre variant 2
mitigations are disabled.

BugLink: https://bugs.launchpad.net/bugs/1822870
Signed-off-by: Diana Craciun <diana.craciun@nxp.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
(backported from commit f633a8ad636efb5d4bba1a047d4a0f1ef719aa06)
Signed-off-by: Manoj Iyer <manoj.iyer@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>