powerpc/5121: move fsl-diu-fb.h to include/linux

Some DIU structures will be used in platform code in
subsequent MPC5121 DIU patch, so we move this header
to be able to include it elsewhere.

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