mpc83xx: Replace ppcDWstore with inline assembly

ppcDWstore/ppcDWload are hardly used by any board, but since they're
implemented in start.S, they're always present in every U-Boot image,
even if they're not needed.

Re-implement these fuctions in C with inline assembly, so that the
compiler can decide when to actually include them.

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