ppc-cpu: simplify MIPS calculation and reduce check frequency

Signed-off-by: Steven Noonan <steven@uplinklabs.net>
ppc-cpu: disable costly assertion

This assertion causes lots of pipeline stalls.

Signed-off-by: Steven Noonan <steven@uplinklabs.net>
configure.ac: default to addressing direct,0x1000

This avoids the annoying vm.mmap_min_addr nonsense.

Signed-off-by: Steven Noonan <steven@uplinklabs.net>