ppc: fix default VGA display for Mac machines

Commit 29f9cef3 "ppc: Include vga cirrus card into the compiling process"
changed the default display adapter for all PPC machines to cirrus. Unfortunately
it missed setting the default display type to stdvga for both Mac machines
causing the display to fail to initialise under OpenBIOS.

Update the MachineClass for both Old World and New World Macs so that the
default std(vga) display adapter is the default if no options are specified
which fixes the display for the Mac machines.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>