powerpc/pseries: Move plpks.h to include directory

Move plpks.h from platforms/pseries/ to include/asm/. This is necessary
for later patches to make use of the PLPKS from code in other subsystems.

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/20230210080401.345462-15-ajd@linux.ibm.com