remoteproc: qcom_q6v5_pas: Deal silently with optional px and cx regulators

Use _get_optional as some platforms might not provide the px
and cx regulators. This avoids printing the following for each
unavailable regulator:

[    4.350229] qcom_q6v5_pas 5c00000.remoteproc: supply cx not found,
using dummy regulator
[    4.374224] qcom_q6v5_pas 5c00000.remoteproc: supply px not found,
using dummy regulator

Signed-off-by: Abel Vesa <abel.vesa@linaro.org>
Acked-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lore.kernel.org/r/20220713152835.3848875-1-abel.vesa@linaro.org