dt-bindings: phy: Add constants for lan966x serdes

Lan966x has: 2 integrated PHYs, 3 SerDes and 2 RGMII interfaces. Which
requires to be muxed based on the HW representation.

So add constants for each interface to be able to distinguish them.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Link: https://lore.kernel.org/r/20211116100818.1615762-3-horatiu.vultur@microchip.com


Signed-off-by: Vinod Koul <vkoul@kernel.org>