phy: armada-38x: add mux value for gbe port 0 on serdes 0

Armada 38x supports 3 functions on serdes #0:
- pcie port 0
- sata port 0
- gbe port 0

Add missing entry for gbe port 0 on serdes 0 to the gbe_mux array.

Because this array looks obscure to new readers, also add a comment
explaining the meaning of rows, columns and values.

Signed-off-by: Josua Mayer <josua@solid-run.com>
Link: https://lore.kernel.org/r/20240106-fix-a38x-comphy-sd0-gbe0-v1-1-c7fd87272050@solid-run.com


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