Autodetect caches and do not flush/invalidate if there are none of them

[fbedard]

Currently OpenOCD always flushes and invalidates caches, even if they are not present. That is mostly ok, because if caches are not present then invalidate/flush commands are ignored. However JTAG cycles are wasted as a result. We need to detect caches presence via BCRs and act accordingly.

ARC OpenOCD already has variable for has_dcaches which allows to avoid flushing, however it can be set to false only manually from configuration scripts. In addition there is no variable to avoid invalidation.