Ensure RegisterBlock memory layout is correct

Without repr(C) the fields may be re-ordered and alignment is
whatever rustc decides is best. repr(C) enforced field ordering
and C alignment rules.

Fixes #13

imxrt-ral/imxrtral.py

@@ -768,6 +768,7 @@ def to_rust_register_block(self):
  address += register.size // 8
  lines = "\n".join(lines)
  return f"""
+ #[repr(C)]
  pub struct RegisterBlock {{
  {lines}
  }}"""