km82xx, km83xx: move ethernet_present() from common to cpu specific

For kmvect1 we need a special solution and for km_arm boards we already
have. So move the common code to the architectur specific file.

Signed-off-by: Karlheinz Jerg <karlheinz.jerg@keymile.com>
Signed-off-by: Holger Brunck <holger.brunck@keymile.com>