phy: core: add PHY_MODE_ETHERNET

Add new PHY's mode to be used by Ethernet PHY interface drivers or
multipurpose PHYs like serdes. It will be reused in further changes.

Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>