MPC8xx: fix build problem for ETX094 board

Commit 7616e785 "Add Ethernet hardware MAC address framework to usbnet"
increased the code size and broke building for the ETX094 board.
Adjust the linker script to make it build again.

While we are at it, remove unused u-boot.lds.debug

Signed-off-by: Wolfgang Denk <wd@denx.de>