powerpc/pseries: Simplify vpa deregistration functions

The VPA, SLB shadow and DTL degistration functions do not need an
address, so simplify things and remove it.

Also cleanup pseries_kexec_cpu_down a bit by storing the cpu IDs
in local variables.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>