dt-bindings: phy: Add sparx5-serdes bindings

Document the Sparx5 ethernet serdes phy driver bindings.

Signed-off-by: Lars Povlsen <lars.povlsen@microchip.com>
Signed-off-by: Steen Hegelund <steen.hegelund@microchip.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20210218161451.3489955-2-steen.hegelund@microchip.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>