dt-bindings: clock: add rpmhcc bindings for sdm670

The Snapdragon 670 uses the RPMh mailbox for some clocks. Document its
support.

Signed-off-by: Richard Acayan <mailingradian@gmail.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20220920223734.151135-2-mailingradian@gmail.com