powerpc: Make sparsemem the default on 64-bit Book3S

Make sparsemem the default on all 64-bit Book3S platforms. It already is
for pseries and ps3, and we need to enable it for powernv because on
POWER9 memory between chips is discontiguous.

For the other platforms sparsemem should work fine, though it might add
a small amount of overhead. We can always force FLATMEM in the
defconfigs if necessary.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>