platform: Make the platform read-only

platform should be a read-only variable, if it is placed in the data
segment, it may be exploited.

Signed-off-by: Xiang Wang <wxjstz@126.com>

platform/template/platform.c

@@ -207,7 +207,7 @@ static int platform_system_shutdown(u32 type)
 /*
  * Platform descriptor.
  */
-struct sbi_platform platform = {
+const struct sbi_platform platform = {
 
 	.name = "platform-name",
 	.features = SBI_PLATFORM_DEFAULT_FEATURES,