Kernel: Implement TimeManagement for aarch64

This sets up the RPi::Timer to trigger an interurpt every 4ms using one
of the comparators. The actual time is calculated by looking at the main
counter of the RPi::Timer using the Timer::update_time function.

A stub for Scheduler::timer_tick is also added, since the TimeManagement
code now calls the function.

Kernel/Arch/aarch64/Dummy.cpp

@@ -15,12 +15,19 @@
 #include <Kernel/PhysicalAddress.h>
 #include <Kernel/Process.h>
 #include <Kernel/Random.h>
+#include <Kernel/Scheduler.h>
 #include <Kernel/Sections.h>
 #include <Kernel/UserOrKernelBuffer.h>
 
 // Scheduler
 namespace Kernel {
 
+void Scheduler::timer_tick(RegisterState const&) {
+    // NOTE: This function currently will be called by the TimeManagement code,
+    //       so there is no TODO_AARCH64. Instead there will be a linker error when
+    //       the Scheduler code is compiled for aarch64.
+};
+
 READONLY_AFTER_INIT Thread* g_finalizer;
 RecursiveSpinlock g_scheduler_lock { LockRank::None };