include: move system timer headers to include/drivers/timer/

Move internal and architecture specific headers from include/drivers to
subfolder for timer:

   include/drivers/timer

Signed-off-by: Anas Nashif <anas.nashif@intel.com>
diff --git a/drivers/timer/native_posix_timer.c b/drivers/timer/native_posix_timer.c
index e15606e..56e2001 100644
--- a/drivers/timer/native_posix_timer.c
+++ b/drivers/timer/native_posix_timer.c
@@ -14,7 +14,7 @@
 #include "zephyr/types.h"
 #include "irq.h"
 #include "device.h"
-#include "drivers/system_timer.h"
+#include <drivers/timer/system_timer.h>
 #include "sys_clock.h"
 #include "timer_model.h"
 #include "soc.h"