integrity/powerpc: Support loading keys from PLPKS

Add support for loading keys from the PLPKS on pseries machines, with the
"ibm,plpks-sb-v1" format.

The object format is expected to be the same, so there shouldn't be any
functional differences between objects retrieved on powernv or pseries.

Unlike on powernv, on pseries the format string isn't contained in the
device tree. Use secvar_ops->format() to fetch the format string in a
generic manner, rather than searching the device tree ourselves.

(The current code searches the device tree for a node compatible with
"ibm,edk2-compat-v1". This patch switches to calling secvar_ops->format(),
which in the case of OPAL/powernv means opal_secvar_format(), which
searches the device tree for a node compatible with "ibm,secvar-backend"
and checks its "format" property. These are equivalent, as skiboot creates
a node with both "ibm,edk2-compat-v1" and "ibm,secvar-backend" as
compatible strings.)

Signed-off-by: Russell Currey <ruscur@russell.cc>
Signed-off-by: Andrew Donnellan <ajd@linux.ibm.com>
Reviewed-by: Stefan Berger <stefanb@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20230210080401.345462-27-ajd@linux.ibm.com