dtc: Refactor printing of property values in -Odts mode
dtc: Refactor printing of property values in -Odts mode

Currently the main recursive tree printing function,
write_tree_source_node(), calls guess_type() to apply heuristics to
see how to print a property value, then calls the appropriate
write_propval_*() function to print it.

However, future heuristics for handling internal labels and the like
don't work well this way.  Therefore, this patch refactors things to
have write_tree_source_node() call a new write_propval() function,
which incorporates the heurstic logic from guess_type() and also calls
the right function to do the actual printing.

No behavioural change.

Signed-off-by: David Gibson <aa743a0aaec8f7d7a1f01442503957f4d7a2d634@gibson.dropbear.id.au>

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