dts: dtlib: Remember the format of assignments

Previously, dtlib just stored the raw 'bytes' value for each property,
along with some markers in Property._markers for phandle and path
references.

Extend Property._markers to also remember where different data blocks
start, so that e.g.

    foo = <1 2 3>, "bar", [00 01];

can be reproduced as written.

Use the new information to reproduce properties as written in
Property.__str__(). This gives good test coverage as well, since the
test suite checks literal __str__() output.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>