dt-bindings: mfd: qcom,tcsr: Add qcom,tcsr-mdm9615

Document the (already used) TCSR on MDM9615.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20220607133443.182468-2-krzysztof.kozlowski@linaro.org