[lk] Add timestamps to dprintf logs for easier performance analysis

Change-Id: I0c4add52d9c6fc2a5e5e50bfee05156371a6632e

lib/debug/debug.c

@@ -31,6 +31,7 @@
 #include <platform.h>
 #include <platform/debug.h>
 #include <kernel/thread.h>
+#include <kernel/timer.h>
 
 void spin(uint32_t usecs)
 {
@@ -70,8 +71,12 @@ int _dputs(const char *str)
 int _dprintf(const char *fmt, ...)
 {
 	char buf[256];
+	char ts_buf[13];
 	int err;
 
+	snprintf(ts_buf, sizeof(ts_buf), "[%u] ", current_time());
+	dputs(ALWAYS, ts_buf);
+
 	va_list ap;
 	va_start(ap, fmt);
 	err = vsnprintf(buf, sizeof(buf), fmt, ap);