[BUG] stringify_rdl_value not implemented for custom properties

[paul-demo]

When trying to parse + re-export SystemRdl description using the command peakrdl systemrdl -o exported.rdl some_file.rdl, I'm finding that the SystemRDL exporter fails due to a NotImplementedError on line 28 here:

PeakRDL-systemrdl/src/peakrdl_systemrdl/stringify.py

Lines 26 to 28 in 92036db

	elif isinstance(value, (node.Node, rdltypes.PropertyReference)):
	# TODO: Throw error. not supported
	raise NotImplementedError

File ".../.venv/lib/python3.9/site-packages/peakrdl_systemrdl/rdl_generator.py", line 38, in assign_properties
value_s = stringify_rdl_value(value)
File ".../.venv/lib/python3.9/site-packages/peakrdl_systemrdl/stringify.py", line 28, in stringify_rdl_value
raise NotImplementedError

The .rdl file in question uses PeakRDL's custom properties for atomic/double-buffered reads, so I suspect that is the issue which is causing this since it's some PeakRDL-specific property that is perhaps not supported by the SystemRDL standard.

property buffer_reads {
    component = reg;
    type = boolean;
};

property rbuffer_trigger {
    component = reg;
    type = ref;
};