powerpc/pmac: Remove early allocation of the SMU command buffer

The SMU command buffer needs to be allocated below 2G using memblock.

In the past, this had to be done very early from the arch code as
memblock wasn't available past that point. That is no longer the
case though, smu_init() is called from setup_arch() when memblock
is still functional these days. So move the allocation to the
SMU driver itself.

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