Use a const pointer for map_to_sysmem()

This function does not actually change the pointer contents, so use const
so that functions which have a const pointer do not need to cast.

Signed-off-by: Simon Glass <sjg@chromium.org>

arch/sandbox/cpu/cpu.c

@@ -58,7 +58,7 @@ void *map_physmem(phys_addr_t paddr, unsigned long len, unsigned long flags)
 	return (void *)(gd->arch.ram_buf + paddr);
 }
 
-phys_addr_t map_to_sysmem(void *ptr)
+phys_addr_t map_to_sysmem(const void *ptr)
 {
 	return (u8 *)ptr - gd->arch.ram_buf;
 }