powerpc/pseries: PLPKS SED Opal keystore support

Define operations for SED Opal to read/write keys
from POWER LPAR Platform KeyStore(PLPKS). This allows
non-volatile storage of SED Opal keys.

Signed-off-by: Greg Joyce <gjoyce@linux.vnet.ibm.com>
Reviewed-by: Jonathan Derrick <jonathan.derrick@linux.dev>
Link: https://lore.kernel.org/r/20231004201957.1451669-4-gjoyce@linux.vnet.ibm.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>