net: macsec: introduce the macsec_context structure

This patch introduces the macsec_context structure. It will be used
in the kernel to exchange information between the common MACsec
implementation (macsec.c) and the MACsec hardware offloading
implementations. This structure contains pointers to MACsec specific
structures which contain the actual MACsec configuration, and to the
underlying device (phydev for now).

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