[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/main.cpp

@@ -169,6 +169,7 @@ kernel_main(args::header *header)
 	*/
 
 	devices.get_lapic()->calibrate_timer();
+	devices.init_drivers();
 
 	syscall_enable();
 	scheduler *sched = new (&scheduler::get()) scheduler(devices.get_lapic());