fbdev: Constify struct sbus_mmap_map

'struct sbus_mmap_map' are not modified in these drivers.

Constifying this structure moves some data to a read-only section, so
increases overall security.

Update sbusfb_mmap_helper() accordingly.

On a x86_64, with allmodconfig, as an example:
Before:
======
   text	   data	    bss	    dec	    hex	filename
   2452	    536	     16	   3004	    bbc	drivers/video/fbdev/bw2.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
   2500	    483	     16	   2999	    bb7	drivers/video/fbdev/bw2.o

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Helge Deller <deller@gmx.de>