device/mdio: Provide helper functions for read and write

This patch provides helper functions to read or write a register via the
MDIO bus. They can be used from drivers to easily access registers on
the MDIO bus.

Change-Id: I293d93435d27269a071b4b9b94a1b55307c575a7
Signed-off-by: Werner Zeh <werner.zeh@siemens.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/69611
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/include/device/mdio.h

@@ -14,4 +14,7 @@ struct mdio_bus_operations {
 /* Helper for getting mdio operations from a device */
 const struct mdio_bus_operations *dev_get_mdio_ops(struct device *dev);
 
+uint16_t mdio_read(struct device *dev, uint8_t offset);
+void mdio_write(struct device *dev, uint8_t offset, uint16_t val);
+
 #endif	/* __DEVICE_MDIO_H__ */