Kernel/aarch64: Add function to MMU.cpp to unmap identity mapping

This function will be used once the kernel runs in high virtual memory
to unmap the identity mapping as userspace will later on use this memory
range instead.

Kernel/Arch/aarch64/CPU.h

@@ -13,6 +13,7 @@ namespace Kernel {
 
 void initialize_exceptions();
 void init_page_tables();
+void unmap_identity_map();
 void panic_without_mmu(StringView);
 void dbgln_without_mmu(StringView);