soc: xtensa: rename the cavstool back for backward compatibility

The name change for cavstool.py has also broken the backward
compatibility for SOF testing. Rename cavstool_server.py back
to cavstool.py. Keep the functionality still as same as the previous
one.

And also update the documentation of it.

Signed-off-by: Enjia Mai <enjia.mai@intel.com>