Kernel: Add RPi Watchdog and use it for system shutdown

The Raspberry Pi hardware doesn't support a proper software-initiated
shutdown, so this instead uses the watchdog to reboot to a special
partition which the firmware interprets as an immediate halt on
shutdown. When running under Qemu, this causes the emulator to exit.

Kernel/Arch/aarch64/RPi/Watchdog.h

@@ -0,0 +1,24 @@
+/*
+ * Copyright (c) 2023, Daniel Bertalan <dani@danielbertalan.dev>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+namespace Kernel::RPi {
+
+struct WatchdogRegisters;
+
+class Watchdog {
+public:
+    static Watchdog& the();
+
+    void system_shutdown();
+
+private:
+    Watchdog();
+
+    WatchdogRegisters volatile* m_registers;
+};
+}