[POWERPC] XilinxFB: Move xilinxfb_platform_data definition to a shared header file

XilnixFB can be used by more than just arch/ppc.  Move the data structure
definition into include/linux/xilinxfb.h so it can be used by microblaze
and arch/powerpc

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>