net: phy: mscc: macsec initialization

This patch adds support for initializing the MACsec engine found within
some Microsemi PHYs. The engine is initialized in a passthrough mode and
does not modify any incoming or outgoing packet. But thanks to this it
now can be configured to perform MACsec transformations on packets,
which will be supported by a future patch.

The MACsec read and write functions are wrapped into two versions: one
called during the init phase, and the other one later on. This is
because the init functions in the Microsemi PHY driver are called while
the MDIO bus lock is taken.

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