mpc83xx: wait till UPM completes the write to array

Reference manual states that MxMR[MAD] increment is the indication
of write to UPM array is complete. Honour that. Also, make the dummy
write explicit.

also fix the comment.

Signed-off-by: Selvamuthukumar <selva.muthukumar@e-coninfotech.com>
Signed-off-by: Kim Phillips <kim.phillips@freescale.com>