dt-bindings: net: marvell,orion-mdio: Drop "reg" sizes schema

Defining the size of register regions is not really in scope of what
bindings need to cover. The schema for this is also not completely correct
as a reg entry can be variable number of cells for the address and size,
but the schema assumes 1 cell.

Signed-off-by: Rob Herring <robh@kernel.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/20231213232455.2248056-1-robh@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>