powerpc/5121: fsl-diu-fb: fix issue with re-enabling DIU area descriptor

On MPC5121e Rev 2.0 re-configuring the DIU area descriptor
by writing new descriptor address doesn't always work.
As a result, DIU continues to display using old area descriptor
even if the new one has been written to the descriptor register of
the plane.

Add the code from Freescale MPC5121EADS BSP for writing descriptor
addresses properly. This fixes the problem for Rev 2.0 silicon.

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>