net: macsec: move some definitions in a dedicated header

This patch moves some structure, type and identifier definitions into a
MACsec specific header. This patch does not modify how the MACsec code
is running and only move things around. This is a preparation for the
future MACsec hardware offloading support, which will re-use those
definitions outside macsec.c.

Signed-off-by: Antoine Tenart <antoine.tenart@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>