powerpc: Replace GPL boilerplate with SPDX identifiers

The FSF does not reside in "675 Mass Ave, Cambridge" anymore...
let's simply use proper SPDX identifiers instead.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Acked-by: Russell Currey <ruscur@russell.cc>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20190828060737.32531-1-thuth@redhat.com