net: phy: mscc: macsec support

This patch adds MACsec offloading support to some Microsemi PHYs, to
configure flows and transformations so that matched packets can be
processed by the MACsec engine, either at egress, or at ingress.

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