of/fdt: Mark initial_boot_params as __ro_after_init

The FDT pointer, i.e. initial_boot_params, shouldn't be changed after
init. It's only set by boot code and then the only user of the FDT is
the raw sysfs reading API. Mark this pointer with __ro_after_init so
that the pointer can't be changed after init.

Cc: Hsin-Yi Wang <hsinyi@chromium.org>
Signed-off-by: Stephen Boyd <swboyd@chromium.org>
Signed-off-by: Rob Herring <robh@kernel.org>

drivers/of/fdt.c

@@ -516,7 +516,7 @@ EXPORT_SYMBOL_GPL(of_fdt_unflatten_tree);
 int __initdata dt_root_addr_cells;
 int __initdata dt_root_size_cells;
 
-void *initial_boot_params;
+void *initial_boot_params __ro_after_init;
 
 #ifdef CONFIG_OF_EARLY_FLATTREE