mpc83xx: Define _end symbol

To support OF_EMBED, the MPC83xx architecture has to define the "_end"
symbol to correctly access the appended DT.

Fortunately, MPC8xx already defines the symbol, and the linker script is
quite similar to that of MPC83xx, so copy this approach for MPC83xx.

Signed-off-by: Mario Six <mario.six@gdsys.cc>