net: phylink: Add phylink_set_fixed_link() to configure fixed link state in phylink

The function allows for the configuration of a fixed link state for a given
phylink instance. This addition is particularly useful for network devices that
operate with a fixed link configuration, where the link parameters do not change
dynamically. By using `phylink_set_fixed_link()`, drivers can easily set up
the fixed link state during initialization or configuration changes.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Russell King <linux@armlinux.org.uk>
Signed-off-by: Raju Lakkaraju <Raju.Lakkaraju@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>