[kernel] Add HPET support, create clock class

Create a clock class which can be queried for current timestamp in
nanoseconds. Also implements a simple HPET class as one possible clock
source.

Tags: time

src/kernel/clock.cpp

@@ -0,0 +1,22 @@
+#include "clock.h"
+
+clock * clock::s_instance = nullptr;
+
+clock::clock(uint64_t rate, clock::source source_func, void *data) :
+	m_rate(rate),
+	m_data(data),
+	m_source(source_func)
+{
+	// TODO: make this atomic
+	if (s_instance == nullptr)
+		s_instance = this;
+	update();
+}
+
+void
+clock::spinwait(uint64_t ns) const
+{
+	uint64_t when = m_source(m_data) + ns;
+	while (value() < when);
+}
+