dt-bindings: phy: Update SERDES_MAX to be SERDES_MAX + 1

SERDES_MAX is a valid value to index ctrl->phys in
drivers/phy/mscc/phy-ocelot-serdes.c. But, currently,
there is an out-of-bounds bug in the mentioned driver
when reading from ctrl->phys, because the size of
array ctrl->phys is SERDES_MAX.

Partially fix this by updating SERDES_MAX to be SERDES6G_MAX + 1.

Notice that this is the first part of the solution to
the out-of-bounds bug mentioned above. Although this
change is not dependent on any other one.

Suggested-by: Quentin Schulz <quentin.schulz@bootlin.com>
Reviewed-by: Quentin Schulz <quentin.schulz@bootlin.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: David S. Miller <davem@davemloft.net>