Remove needless debug handler state.

 - "handler_installed" became wrong as soon as the second TRST+SRST
   reset was issued ... so the handler was never reloaded after the
   reset removed it from the mini-icache.

   This fixes the bug where subsequent resets fail on PXA255 (if the
   first one even worked, which is uncommon).  Other XScale chips
   would have problems too; PXA270 seems to have, IXP425 maybe not.

 - "handler_running" was never tested; it's pointless.

Plus a related bugfix: invalidate OpenOCD's ARM register cache on reset.
It was no more valid than the XScale's mini-icache.  (Though ... such
invalidations might be better done in "SRST asserted" callbacks.)

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>