dt-bindings: remoteproc: qcom,wcnss-pil: Add support for single power-domain platforms

dt-bindings: remoteproc: qcom,wcnss-pil: Add support for single power-domain platforms

Support platforms such as MSM8226 and MSM8974 with only one power rail
(CX) modelled as power domain while MX and PX are regulators.

[luca: reword commit message, expand based on feedback from
 Stephan Gerhold]

Signed-off-by: Matti Lehtimäki <matti.lehtimaki@gmail.com>
Signed-off-by: Luca Weiss <luca@lucaweiss.eu>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20250206-wcnss-singlepd-v2-1-9a53ee953dee@lucaweiss.eu
Signed-off-by: Bjorn Andersson <andersson@kernel.org>