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>

include/phy.h

@@ -568,14 +568,6 @@ int phy_xilinx_gmii2rgmii_init(void);
 int board_phy_config(struct phy_device *phydev);
 int get_phy_id(struct mii_dev *bus, int addr, int devad, u32 *phy_id);
 
-/**
- * phy_get_interface_by_name() - Look up a PHY interface name
- *
- * @str:	PHY interface name, e.g. "mii"
- * @return: PHY_INTERFACE_MODE_... value, or -1 if not found
- */
-int phy_get_interface_by_name(const char *str);
-
 /**
  * phy_interface_is_rgmii - Convenience function for testing if a PHY interface
  * is RGMII (all variants)