docs: dt: minor adjustments at writing-schema.rst

docs: dt: minor adjustments at writing-schema.rst

There are two literal blocks that aren't mark as such. Mark them,
in order to make the document to produce a better html output.

While here, also add a SPDX header to it.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/14fc680fd6596b277f94bb5a240cc9dfc41d59bf.1592203542.git.mchehab+huawei@kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>