Rework management of the external L2 cache on the few Indy/Indigo2 systems

Rework management of the external L2 cache on the few Indy/Indigo2 systems
which have it.

Instead of implementing external L2 maintainance at the cache routine level,
let bus_dmamap_sync(9) know about the possible existence of an external L2,
and invoke a dedicated routine to perform the necessary cache operations.

This way, the external L2 dmamap_sync function pointer can get invoked with
the physical address to operate on; this saves the pmap_extract() calls the
previous cache routine had to do.