powerpc/pseries: Add helper to get PLPKS password length

Add helper function to get the PLPKS password length. This will be used
in a later patch to support passing the password between kernels over
kexec.

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/20230210080401.345462-23-ajd@linux.ibm.com