powerpc/mpc5121: correct DIU compatible property

The DIU driver should bind against "fsl,mpc5121-diu"
directly. Add this compatible property to the match
table and fix DTS and platform code accordingly.

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