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