dt-bindings: mailbox: qcom: add one more clock provider for IPQ mailbox

Mailbox controller present in the IPQ SoCs takes the GPLL0 clock also as
an input. Document the same.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Kathiravan Thirumoorthy <quic_kathirav@quicinc.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>