pub fn fields(
fields: Vec<&Field>,
regspec_ty: &Ident,
rmwv: Option<ModifiedWriteValues>,
access: Access,
properties: &RegisterProperties,
mod_items: &mut TokenStream,
rpath: &RegisterPath,
index: &Index<'_>,
config: &Config
) -> Result<(TokenStream, TokenStream, u64, u64)>