Switched to UPPER_CASE for unions

PascalCase doesn't work when we have registers named
`FOO_11` and `FOO1_1`.

This happened in
* Freescale MKL81Z7
* Freescale MKL82Z7
* Freescale MK81F25615
* Freescale MK82F25615
on `LTC0_PK*` registers.

Author: Emilgardis

src/generate/peripheral.rs

@@ -490,7 +490,7 @@ fn register_or_cluster_block_nightly(
         if region.fields.len() > 1 && !block_is_union {
             let (type_name, name) = match region.ident.clone() {
                 Some(prefix) => {
-                    (Ident::new(format!("{}Union", prefix.to_sanitized_pascal_case())),
+                    (Ident::new(format!("{}Union", prefix.to_sanitized_upper_case())),
                     Ident::new(prefix))
                 }
                 // If we can't find a name, fall back to the region index as a