dt-bindings: phy: add cpsw port interface mode selection phy bindings

Add CPSW Port's Interface Mode Selection PHY (phy-gmii-sel) DT Bindings

Cc: Kishon Vijay Abraham I <kishon@ti.com>
Cc: Tony Lindgren <tony@atomide.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>