dt-bindings: cpufreq: qcom-cpufreq-nvmem: make cpr bindings optional

The qcom-cpufreq-nvmem driver supports 2 kind of devices:
- pre-cpr that doesn't have power-domains and base everything on nvmem
  cells and multiple named microvolt bindings.
  Doesn't need required-opp binding in the opp nodes as they are only
  used for genpd based devices.
- cpr-based that require power-domain in the cpu nodes and use various
  source to decide the correct voltage and freq
  Require required-opp binding since they need to be linked to the
  related opp-level.

When the schema was introduced, it was wrongly set to always require these
binding but this is not the case for pre-cpr devices.

Make the power-domain and the required-opp optional and set them required
only for qcs404 based devices.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>