dt-bindings: power: avs: Add support for CPR (Core Power Reduction)

Add DT bindings to describe the CPR HW found on certain Qualcomm SoCs.

Changes since RFC:
-Make compatible string SoC specific.
-Changed interrupt definition.
-Use clock binding for reference clock.
-Clarified qcom,vdd-apc-step-up-limit description.

Co-developed-by: Jorge Ramirez-Ortiz <jorge.ramirez-ortiz@linaro.org>
Signed-off-by: Jorge Ramirez-Ortiz <jorge.ramirez-ortiz@linaro.org>
Signed-off-by: Niklas Cassel <niklas.cassel@linaro.org>