Kernel/riscv64: Add a Timer class for RISC-V

This is a basic Timer class based on the aarch64 RPi Timer.
It uses the hart-local timer, as defined by the privileged ISA.

Kernel/Arch/riscv64/Timer.h

@@ -0,0 +1,61 @@
+/*
+ * Copyright (c) 2023, Sönke Holz <sholz8530@gmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Types.h>
+#include <Kernel/Interrupts/IRQHandler.h>
+#include <Kernel/Library/NonnullLockRefPtr.h>
+#include <Kernel/Time/HardwareTimer.h>
+
+namespace Kernel::RISCV64 {
+
+struct TimerRegisters;
+
+class Timer final : public HardwareTimer<IRQHandler> {
+public:
+    virtual ~Timer();
+
+    static NonnullLockRefPtr<Timer> initialize();
+
+    virtual HardwareTimerType timer_type() const override { return HardwareTimerType::RISCVTimer; }
+    virtual StringView model() const override { return "RISC-V Timer"sv; }
+    virtual size_t ticks_per_second() const override { return m_frequency; }
+
+    virtual bool is_periodic() const override { TODO_RISCV64(); }
+    virtual bool is_periodic_capable() const override { TODO_RISCV64(); }
+    virtual void set_periodic() override { TODO_RISCV64(); }
+    virtual void set_non_periodic() override { TODO_RISCV64(); }
+    virtual void disable() override { TODO_RISCV64(); }
+
+    virtual void reset_to_default_ticks_per_second() override { TODO_RISCV64(); }
+    virtual bool try_to_set_frequency(size_t) override { TODO_RISCV64(); }
+    virtual bool is_capable_of_frequency(size_t) const override { TODO_RISCV64(); }
+    virtual size_t calculate_nearest_possible_frequency(size_t) const override { TODO_RISCV64(); }
+
+    // FIXME: Share code with HPET::update_time
+    u64 update_time(u64& seconds_since_boot, u32& ticks_this_second, bool query_only);
+
+    u64 microseconds_since_boot();
+
+    void set_interrupt_interval_usec(u32);
+    void enable_interrupt_mode();
+
+private:
+    Timer();
+
+    void set_compare(u64 compare);
+
+    //^ IRQHandler
+    virtual bool handle_irq(RegisterState const&) override;
+
+    u32 m_interrupt_interval { 0 };
+
+    u64 m_main_counter_last_read { 0 };
+    u64 m_main_counter_drift { 0 };
+};
+
+}