ARM: dts: imx6dl-yapp4: Fix RGMII connection to QCA8334 switch

The FEC does not have a PHY so it should not have a phy-handle. It is
connected to the switch at RGMII level so we need a fixed-link sub-node
on both ends.

This was not a problem until the qca8k.c driver was converted to PHYLINK
by commit b3591c2a ("net: dsa: qca8k: Switch to PHYLINK instead of
PHYLIB"). That commit revealed the FEC configuration was not correct.

Fixes: 87489ec3 ("ARM: dts: imx: Add Y Soft IOTA Draco, Hydra and Ursa boards")
Cc: stable@vger.kernel.org
Signed-off-by: Michal Vokáč <michal.vokac@ysoft.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>