mpc83xx: Use pre-defined asm functions

mpc83xx: Use pre-defined asm functions

For a lot of inline assembly calls in the mpc8xxx and mpc83xx
directories, we already have convenient pre-defined helper functions,
but they're not used, resulting in hard-to-read code.

Use these helper functions where ever possible and useful.

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