ppc/km: convert KM_DEF_NETDEV to Kconfig

Remove this from the board header files and move it to Kconfig. Also use
the correct default address for kmtegr1.

Signed-off-by: Holger Brunck <holger.brunck@ch.abb.com>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>
CC: Priyanka Jain <priyanka.jain@nxp.com>
CC: Valentin Longchamp <valentin.longchamp@ch.abb.com>