Kernel/riscv64: Make the kernel compile

This commits inserts TODOs into all necessary places to make the kernel
compile on riscv64!

Kernel/Arch/riscv64/PowerState.cpp

@@ -0,0 +1,21 @@
+/*
+ * Copyright (c) 2023, Sönke Holz <sholz8530@gmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <Kernel/Arch/PowerState.h>
+
+namespace Kernel {
+
+void arch_specific_reboot()
+{
+    TODO_RISCV64();
+}
+
+void arch_specific_poweroff()
+{
+    TODO_RISCV64();
+}
+
+}