net: phy: Introduce phy related fwnode functions

Define fwnode_phy_find_device() to iterate an mdiobus and find the
phy device of the provided phy fwnode. Additionally define
device_phy_find_device() to find phy device of provided device.

Define fwnode_get_phy_node() to get phy_node using named reference.

Signed-off-by: Calvin Johnson <calvin.johnson@oss.nxp.com>
Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Acked-by: Grant Likely <grant.likely@arm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>