Optimize isp(4) to reduce CPU usage, especially in target mode:
 - Remove two excessive and slow register reads from isp_intr().  Instead
of rereading value every time, assume that registers contain what we have
written there.
 - Avoid sequential search through 4096 array elements when looking for
command tag.  Use hash of lists to store active tags separately from free
ones and so greatly speedup the searches.

Reviewed by:	mjacob
- Switch to use WBWA mappings for page tables on armv6, this is needed for SMP.
- Fix PTE_SYNC() for PIPT L2 caches, using the virtual address wasn't so useful.
- Use PTE_SYNC() for >= armv6