powerpc/pseries: Honour current SMT state when DLPAR onlining CPUs

Integrate with the generic SMT support, so that when a CPU is DLPAR
onlined it is brought up with the correct SMT mode.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230705145143.40545-11-ldufour@linux.ibm.com