MAINTAINERS: dt-bindings: Add Krzysztof Kozlowski

Add Krzysztof Kozlowski as a second maintainer for the Devicetree
bindings, to share the Rob's workload and help in review.

Cc: devicetree@vger.kernel.org
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220204161851.138874-1-krzysztof.kozlowski@canonical.com