[IMP] l10n_es_aeat: Avoid rewrites on related fields
[IMP] l10n_es_aeat: Avoid rewrites on related fields

ORM launches a rewrite on origin table for related fields without
readonly flag. Preventing this marking it as readonly, we will
gain a bit in performance or avoid write errors.

RamonGuiuGou/l10n-spain,RamonGuiuGou/l10n-spain