MAINTAINERS: dt-bindings: update Krzysztof Kozlowski's email

Use Krzysztof Kozlowski's @kernel.org account in dt-bindings maintainer
entry.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220307172901.156929-1-krzysztof.kozlowski@canonical.com