net: phy: Add phy_standalone sysfs entry

Export a phy_standalone device attribute that is meant to give the
indication that this PHY lacks an attached_dev and its corresponding
sysfs link. The attribute will be created only when the
phy_attach_direct() function will be called with a NULL net_device.

Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>