Disable default slave for Narrow AXI Xbar #4
Description
Currently, all AXI crossbars in occamy are configured with master port 0 as default slave:
https://github.com/pulp-platform/snitch/blob/49346c18504c06df088bf86a21e78f7ebc31d9ea/util/solder/solder.py#L1364-L1365
For the top-level crossbars, it might make sense to disable the default slave and return a DEC_ERROR when accessing an unmapped memory region instead.