phy: sparx5-serdes: add function for getting the CMU index

The SERDES to CMU mapping is different on Sparx5 and lan969x. Therefore
create a function for getting the CMU index on Sparx5.

Signed-off-by: Daniel Machon <daniel.machon@microchip.com>
Reviewed-by: Steen Hegelund <Steen.Hegelund@microchip.com>
Link: https://lore.kernel.org/r/20240909-sparx5-lan969x-serdes-driver-v2-5-d695bcb57b84@microchip.com


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