eth: Move IPv4/IPv6 multicast address bases to their own symbols

As these addresses can be useful outside of checking if an address
is a multicast address (for example in device drivers) make them
accessible to users of etherdevice.h to avoid code duplication.

Signed-off-by: Diogo Ivo <diogo.ivo@siemens.com>
Reviewed-by: MD Danish Anwar <danishanwar@ti.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>