net: introduce helpers to get PHY interface mode from a device/ofnode

Add helpers ofnode_read_phy_mode() and dev_read_phy_mode() to parse the
"phy-mode" / "phy-connection-type" property. Add corresponding UT test.

Use them treewide.

This allows us to inline the phy_get_interface_by_name() into
ofnode_read_phy_mode(), since the former is not used anymore.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>
Tested-by: Patrice Chotard <patrice.chotard@foss.st.com>

drivers/core/read.c

@@ -403,3 +403,8 @@ ofnode dev_get_phy_node(const struct udevice *dev)
 {
 	return ofnode_get_phy_node(dev_ofnode(dev));
 }
+
+phy_interface_t dev_read_phy_mode(const struct udevice *dev)
+{
+	return ofnode_read_phy_mode(dev_ofnode(dev));
+}