[POWERPC] Split out vpa unregister logic from pseries_kexec_cpu_down_xics()

As part of the new irq code pseries_kexec_cpu_down() was split into a
xics and mpic version. The vpa unregister logic is now only done in the
xics routine, and although that's ok in practice (we don't have SPLPAR
machines with mpic), I'd rather have the two concepts stay separate.

So move the vpa unregister into pseries_kexec_cpu_down(), which gets called
by both the xics and mpic routines. This also gives us an obvious place to
put any new kexec-down logic needed in future.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>