integrity: PowerVM support for loading third party code signing keys

On secure boot enabled PowerVM LPAR, third party code signing keys are
needed during early boot to verify signed third party modules. These
third party keys are stored in moduledb object in the Platform
KeyStore (PKS).

Load third party code signing keys onto .secondary_trusted_keys keyring.

Signed-off-by: Nayna Jain <nayna@linux.ibm.com>
Reviewed-and-tested-by: Mimi Zohar <zohar@linux.ibm.com>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Tested-by: Nageswara R Sastry <rnsastry@linux.ibm.com>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>