hw/ppc/Kconfig: Add dependency PEGASOS2 -> ATI_VGA

hw/ppc/Kconfig: Add dependency PEGASOS2 -> ATI_VGA

The ATI VGA device isn't a requisite for the Pegasos2 machine
because Linux only uses the serial console; see commit ba7e5ac18e7
("hw/ppc: Add emulation of Genesi/bPlan Pegasos II") for rationale.

Using the default devices we don't have any problem:

  $ qemu-system-ppc -M pegasos2
  qemu-system-ppc: standard VGA not available

But when trying to explicitly use the ATI device we get an error:

  $ qemu-system-ppc -M pegasos2 -vga none -bios pegasos2.rom -device ati-vga,romfile=
  qemu-system-ppc: -device ati-vga,romfile=: 'ati-vga' is not a valid device model name

Add it as an implicit Kconfig dependency.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20210515173716.358295-13-philmd@redhat.com>
Acked-by: BALATON Zoltan <balaton@eik.bme.hu>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: David Gibson <david@gibson.dropbear.id.au>