dt-bindings: input: qcom,pm8921-pwrkey: convert to dt-schema

Convert input/qcom,pm8xxx-pwrkey.txt to YAML, and take in account that
the PM8921 pwrkey compatible is used as fallback for the PM8018 pwrkey.

Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://lore.kernel.org/r/20220928-mdm9615-dt-schema-fixes-v4-6-dac2dfaac703@linaro.org


Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>