docs: dt: submitting-patches: drop outdated points to TXT format

docs: dt: submitting-patches: drop outdated points to TXT format

New bindings in TXT format are not accepted and DT schema format expects
all compatibles to be explicitly defined, thus guidance about "wildcard
<chip>" is not correct anymore.  Drop that paragraph and update one more
place which still mentions TXT files.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20240108083750.16350-1-krzysztof.kozlowski@linaro.org
Signed-off-by: Rob Herring <robh@kernel.org>