remoteproc: qcom_q6v5_mss: Handle platforms with one power domain

remoteproc: qcom_q6v5_mss: Handle platforms with one power domain

For example MSM8974 has mx voltage rail exposed as regulator and only cx
voltage rail is exposed as power domain. This power domain (cx) is
attached internally in power domain and cannot be attached in this driver.

Fixes: 8750cf392394 ("remoteproc: qcom_q6v5_mss: Allow replacing regulators with power domains")
Co-developed-by: Matti Lehtimäki <matti.lehtimaki@gmail.com>
Signed-off-by: Matti Lehtimäki <matti.lehtimaki@gmail.com>
Reviewed-by: Stephan Gerhold <stephan.gerhold@linaro.org>
Signed-off-by: Luca Weiss <luca@lucaweiss.eu>
Link: https://lore.kernel.org/r/20250217-msm8226-modem-v5-4-2bc74b80e0ae@lucaweiss.eu
Signed-off-by: Bjorn Andersson <andersson@kernel.org>