net: dsa: hellcreek: convert to phylink_generic_validate()

Populate the supported interfaces and MAC capabilities for the
hellcreek DSA switch and remove the old validate implementation to
allow DSA to use phylink_generic_validate() for this switch driver.

The switch actually only supports MII and RGMII, but as phylib defaults
to GMII, we need to include this interface mode to keep existing DT
working.

Reviewed-by: Kurt Kanzenbach <kurt@linutronix.de>
Tested-by: Kurt Kanzenbach <kurt@linutronix.de>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>