powerpc/64: Setup KUP on secondary CPUs

Some platforms (i.e. Radix MMU) need per-CPU initialisation for KUP.

Any platforms that only want to do KUP initialisation once
globally can just check to see if they're running on the boot CPU, or
check if whatever setup they need has already been performed.

Note that this is only for 64-bit.

Signed-off-by: Russell Currey <ruscur@russell.cc>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>