powerpc/85xx: introduce corenet_generic machine

In the current kernel, the board files for p2041rdb, p3041ds, p4080ds,
p5020ds, p5040ds, t4240qds and b4qds are almost the same except the
machine name. So this introduces a cornet_generic machine to support
all these boards to avoid the code duplication.

With these changes the file corenet_ds.h becomes useless. Just delete
it.

Signed-off-by: Kevin Hao <haokexin@gmail.com>
Signed-off-by: Scott Wood <scottwood@freescale.com>