[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/include/log_areas.inc

@@ -10,3 +10,5 @@ LOG(boot,   debug);
 LOG(syscall,debug);
 LOG(vmem,   debug);
 LOG(objs,   debug);
+LOG(timer,  debug);
+LOG(clock,  debug);