Of the 64-bit Book3S platforms, only powermac supports booting on an
actual non-SMP system. The other platforms can be built with SMP
disabled, but it doesn't make a lot of sense given the CPUs they support
are all multicore or multithreaded.

So give platforms the option of forcing SMP=y.
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>