fsl_upm: Add MxMR/MDR synchronization

fsl_upm: Add MxMR/MDR synchronization

According to Freescale reference manuals (eg section "13.4.4.2
Programming the UPMs" of the P4080 Reference Manual):

"Since the result of any update to the MxMR/MDR register must be in
effect before the dummy read or write to the UPM region, a write to
MxMR/MDR should be followed immediately by a read of MxMR/MDR."

The UPM on a custom P4080-based board did not work without performing
a read of MxMR/MDR after a write.

Signed-off-by: John Schmoller <[email protected]>
Signed-off-by: Peter Tyser <[email protected]>
Acked-by: Scott Wood <[email protected]>
Signed-off-by: Kumar Gala <[email protected]>