net: phy: add Lynx PCS module

Add a Lynx PCS module which exposes the necessary operations to drive
the PCS using phylink.

The majority of the code is extracted from the Felix DSA driver, which
will be also modified in a later patch, and exposed as a separate module
for code reusability purposes.
As such, this aims at feature and bug parity with the existing Felix DSA
driver, and thus USXGMII, SGMII, QSGMII and 2500Base-X (only w/o in-band
AN) are supported by the Lynx PCS module since these were also supported
by Felix.

The module can only be enabled by the drivers in need and not user
selectable.

Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>