powerpc/5121: doc/dts-bindings: update doc of FSL DIU bindings

Update compatible and interrupt properties description.
Furthermore an example for the MPC5121 has been added.

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Acked-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>