dt-bindings: remoteproc: qcom,msm8916-mss-pil: Support platforms with one power domain

Add support for platforms such as MSM8974 which have the mx voltage rail
exposed as regulator and only cx voltage rail as power domain.

Signed-off-by: Matti Lehtimäki <matti.lehtimaki@gmail.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Luca Weiss <luca@lucaweiss.eu>
Link: https://lore.kernel.org/r/20250217-msm8226-modem-v5-1-2bc74b80e0ae@lucaweiss.eu


Signed-off-by: Bjorn Andersson <andersson@kernel.org>