pseries: Add 2.8 machine type, set up compatibility macros

Now that 2.7 is released, create the pseries-2.8 machine type and add the
boilerplate compatiblity macro stuff.  There's nothing new to put into the
2.7 compatiliby properties yet, but we'll need something eventually, so
we might as well get it ready now.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>