config: imx: Enable CONFIG_PHY_FIXED on HSC and DDC i.MX53 boards

The CONFIG_PHY_FIXED is necessary to allow DSA switch work in U-Boot after
the
commit 3bf135b6 ("drivers: net: phy: Ignore PHY ID 0 during PHY probing").

This particular device - LAN9303 - returns phy_id == 0. With
CONFIG_PHY_FIXED enabled HSC and DDC boards work again with the same U-Boot
binary.

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