target-ppc: Remove vestigial PowerPC 620 support

target-ppc: Remove vestigial PowerPC 620 support

The PowerPC 620 was the very first 64-bit PowerPC implementation, but
hardly anyone ever actually used the chips.  qemu notionally supports the
620, but since we don't actually have code to implement the segment table,
the support is broken (quite likely in other ways too).

This patch, therefore, removes all remaining pieces of 620 support, to
stop it cluttering up the platforms we actually care about.  This includes
removing support for the ASR register, used only on segment table based
machines.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Alexander Graf <agraf@suse.de>