Preserve datatype markers when emitting dts format

If datatype markers are present in the property value, use them to
output the data in the correct format instead of trying to guess the
datatype. This also will preserve data grouping, such as in an
interrupts list.

This is a step forward for preserving and using datatype information
when processing DTS/DTB files. Schema validation tools can use the
datatype information to make sure a DT is correctly formed and
intepreted.

Signed-off-by: Grant Likely <grant.likely@arm.com>
[robh: rework marker handling and fix label output]
Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>