powerpc: add security.config, enforcing lockdown=integrity

It's sometimes handy to have a config that boots a bit like a system
under secure boot (forcing lockdown=integrity, without needing any
extra stuff like a command line option).

This config file allows that, and also turns on a few assorted security
and hardening options for good measure.

Suggested-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Daniel Axtens <dja@axtens.net>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20201203042807.1293655-1-dja@axtens.net