dt-bindings: clock: qcom,rpmh: Add CXO PAD clock IDs

The SM8550 has a new fixed divider as child clock of CXO
called CXO_PAD, so add IDs for it.

Signed-off-by: Abel Vesa <abel.vesa@linaro.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>