apm821xx: bring back AT803X_PHY
AT803X_PHY was replaced with QCA83XX_PHY which handles external switches.
But for internal ones like in the Meraki MR24, AT803X_PHY is needed.
Signed-off-by: Rosen Penev <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/16737
Signed-off-by: Robert Marko <[email protected]>