ppc: fix default VGA display for PReP 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 PReP machines
causing the display to fail to initialise under OpenHackWare.

Update the MachineClass for both prep and 40p machines so that the default
std(vga) display adapter is the default if no options are specified
which fixes the display for the PReP machines.

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