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.

Change-Id: I01db1957e2cfad38f3c37ab29b1276a9daaa7c6c
Signed-off-by: Antoine Tenart <antoine.tenart@bootlin.com>
Git-commit: befde3d5a01cb28ca7c5e6d5230557325b112a10
Git-repo: https://github.com/aquantia/linux-4.14-atlantic-forwarding


Signed-off-by: Jinesh K. Jayakumar <jineshk@codeaurora.org>