powerpc/pseries: Implement signed update for PLPKS objects

The Platform Keystore provides a signed update interface which can be used
to create, replace or append to certain variables in the PKS in a secure
fashion, with the hypervisor requiring that the update be signed using the
Platform Key.

Implement an interface to the H_PKS_SIGNED_UPDATE hcall in the plpks
driver to allow signed updates to PKS objects.

(The plpks driver doesn't need to do any cryptography or otherwise handle
the actual signed variable contents - that will be handled by userspace
tooling.)

Signed-off-by: Nayna Jain <nayna@linux.ibm.com>
[ajd: split patch, add timeout handling and misc cleanups]
Co-developed-by: Andrew Donnellan <ajd@linux.ibm.com>
Signed-off-by: Andrew Donnellan <ajd@linux.ibm.com>
Signed-off-by: Russell Currey <ruscur@russell.cc>
Reviewed-by: Stefan Berger <stefanb@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20230210080401.345462-18-ajd@linux.ibm.com