psci: MAINTAINERS: Include DT binding in the entry

Updates to PSCI Devicetree binding seems to be not picked up from the
mailing list, probably because actual maintainers are not CC-ed.  Add
the binding to the PCSI maintainers entry.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20250501150934.77317-4-krzysztof.kozlowski@linaro.org


Signed-off-by: Rob Herring (Arm) <robh@kernel.org>