VAX8200: Fix access to watch chip wtc_rd() and wtc_wr()

- Recent changes in vax_watch.c changed the input parameter from a
  physical address to a register address within the watch chip.
- vax630_sysdev.c called the older wtc_wr() with the new parameter,
  but the length argument has been removed.
- The watch chip never starts as valid.  Make it valid when the TODR is
  attached (i.e. reflecting a connected battery).
- 64 bytes of bus addresses for the watch chip can sometimes be dispatched
  to wtc_rd() and wtc_wr().  Make sure that reasonable memory is always
  referenced for all potential accesses.
- restore vax8200 to makefile which got lost in a prior merge.

VAX/vax820_syslist.c

@@ -43,6 +43,7 @@ extern DEVICE ka_dev[KA_NUM];
 extern DEVICE mctl_dev[MCTL_NUM];
 extern DEVICE uba_dev;
 extern DEVICE clk_dev;
+extern DEVICE wtc_dev;
 extern DEVICE tmr_dev;
 extern DEVICE tti_dev, tto_dev;
 extern DEVICE fl_dev;
@@ -77,6 +78,7 @@ DEVICE *sim_devices[] = {
     &mctl_dev[1],
     &uba_dev,
     &clk_dev,
+    &wtc_dev,
     &tmr_dev,
     &tti_dev,
     &tto_dev,