ppc: Remove corenet_ds boards

These boards have been orphaned for some time and are behind on various
DM migrations. Remove them.

Cc: Priyanka Jain <priyanka.jain@nxp.com>
Cc: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Tom Rini <trini@konsulko.com>