Order the CONFIG switches in ppc-softmmu.mak according to the machine
classes where they are used (embedded, Mac or PReP), so that it is
easier for the users to disable a set of switches completely if they
are not needed.

Also add the missing CONFIG_IDE_SII3112 switch to the embedded section
which was previously only added to ppcemb-softmmu.mak.

And while we're at it, also remove the CONFIG_IDE_CMD646 switch since
this controller does not seem to be used by any ppc machine in QEMU.
Signed-off-by: NThomas Huth <thuth@redhat.com>
Signed-off-by: NDavid Gibson <david@gibson.dropbear.id.au>