net: phy: fixed_phy: remove link gpio support

net: phy: fixed_phy: remove link gpio support

The only user of fixed_phy gpio functionality was here:
arch/arm/boot/dts/nxp/vf/vf610-zii-dev-rev-b.dts
Support for the switch on this board was migrated to phylink
(DSA - mv88e6xxx) years ago, so the functionality is unused now.
Therefore remove it.

Note: There is a very small risk that there's out-of-tree users
who use link gpio with a switch chip not handled by DSA.
However we care about in-tree device trees only.

Suggested-by: Russell King (Oracle) <linux@armlinux.org.uk>
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Link: https://patch.msgid.link/75295a9a-e162-432c-ba9f-5d3125078788@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>