target-ppc: Add MMU model check for booke machines

Machines bamboo, e500 and virtex-ml507 assume a certain MMU model,
otherwise resulting in unpredictable behavior. Add apropriate checks
into *_init functions.

Signed-off-by: Valentin Plotkin <caliborn@sdf.org>

[regarding virtex parts]
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Tested-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>

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