mac-io: Add escc-legacy memory alias region

mac-io: Add escc-legacy memory alias region

Mac OS X's debugging serial driver accesses the ESCC through a different
register layout, called "escc-legacy". This layout differs from the normal
escc register layout purely by the location of the respective registers.

This patch adds a memory alias region that takes normal escc registers and
maps them into the escc-legacy register space.

With this patch applied, a Mac OS X guest successfully emits debug output
on the serial port when run with debug parameters set, for example by running:

  $ qemu-system-ppc -prom-env -'boot-args=-v debug=0x8 io=0xff serial=0x3' \
                    -cdrom 10.4.iso -boot d

Signed-off-by: Alexander Graf <agraf@suse.de>