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 3bf135b6
("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>