dts: imx: Add fixed-link property to HSC and DDC (imx53) devices

Those two boards are supposed to be run with a single u-boot binary.
There are notable differences though - HSC uses DSA switch (which
phy_id == 0x0) and DCC (DP83848C).

After the commit 3bf135b6c3
("drivers: net: phy: Ignore PHY ID 0 during PHY probing") the PHY devices
with phy_id == 0 are not created in U-Boot anymore. This caused regression
on HSC.

To fix this problem - the fec's 'fixed-link' node has been introduced and
the phy_id is not assessed anymore. This approach works on both boards.

Signed-off-by: Lukasz Majewski <lukma@denx.de>

arch/arm/dts/imx53-kp-u-boot.dtsi

@@ -5,6 +5,13 @@
  * SPDX-License-Identifier:     GPL-2.0+ or X11
  */
 
+&fec {
+	fixed-link { /* RMII fixed link for both HSC|DDC */
+		speed = <100>;
+		full-duplex;
+	};
+};
+
 &pmic {
 	u-boot,i2c-transaction-bytes = <3>;
 };