net: introduce MACsec ops and add a reference in net_device

This patch introduces MACsec ops for drivers to support offloading
MACsec operations. A reference to those ops is added in net_device.

Change-Id: Iddf4947e4a0a8011d3305af1ec816990656759ec
Signed-off-by: Antoine Tenart <antoine.tenart@bootlin.com>
Git-commit: e7656949ab16103b6ed85d9ab416c7802471725a
Git-repo: https://github.com/aquantia/linux-4.14-atlantic-forwarding


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