powerpc/64: Move security code into security.c

When the original spectre/meltdown mitigations were merged we put them
in setup_64.c for lack of a better place.

Since then we created security.c for some of the other mitigation
related code. But it should all be in there.

This sort of code movement can cause trouble for backports, but
hopefully this code is relatively stable these days (famous last words).

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210326101201.1973552-1-mpe@ellerman.id.au