dt-bindings: net: altera: tse: add an optional pcs register range

Some implementations of the TSE have their PCS as an external bloc,
exposed at its own register range. Document this, and add a new example
showing a case using the pcs and the new phylink conversion to connect
an sfp port to a TSE mac.

Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>