net: dsa: Add PHYLINK switch operations

In preparation for adding support for PHYLINK within DSA, define a number of
operations that we will need and that switch drivers can start implementing.
Proper integration with PHYLINK will follow in subsequent patches.

We start selecting PHYLINK (which implies PHYLIB) in net/dsa/Kconfig
such that drivers can be guaranteed that this dependency is properly
taken care of and can start referencing PHYLINK helper functions without
requiring stubs or anything.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>