Added derivation of TDF base class, for AVR8 targets.

Includes initialisation and validation code.

build/scripts/TargetDescriptionFiles/RegisterGroup.php

@@ -0,0 +1,13 @@
+<?php
+namespace Bloom\BuildScripts\TargetDescriptionFiles;
+
+require_once __DIR__ . "/Register.php";
+
+class RegisterGroup
+{
+    public ?string $name = null;
+    public ?int $offset = null;
+
+    /** @var Register[] */
+    public array $registersMappedByName = [];
+}