Kernel/riscv64: Add MMU initialization code

We initialize the MMU by first setting up the page tables for the
kernel image and the initial kernel stack.
Then we jump to a identity mapped page which makes the newly created
kernel root page table active by setting `satp` and then jumps to
`init`.

Kernel/Arch/riscv64/MMU.h

@@ -0,0 +1,18 @@
+/*
+ * Copyright (c) 2023, Sönke Holz <sholz8530@gmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Forward.h>
+
+#include <AK/Platform.h>
+VALIDATE_IS_RISCV64()
+
+namespace Kernel::Memory {
+
+[[noreturn]] void init_page_tables_and_jump_to_init();
+
+}