pmdomain: qcom: rpmhpd: Add MXC to SC8280XP

pmdomain: qcom: rpmhpd: Add MXC to SC8280XP

This was apparently accounted for in dt-bindings, but never made its
way into the driver.

Fix it for SC8280XP and its VDD_GFX-less cousin, SA8540P.

Fixes: f68f1cb3437d ("soc: qcom: rpmhpd: add sc8280xp & sa8540p rpmh power-domains")
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Signed-off-by: Konrad Dybcio <konrad.dybcio@oss.qualcomm.com>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Link: https://lore.kernel.org/r/20251202-topic-8280_mxc-v2-2-46cdf47a829e@oss.qualcomm.com
Signed-off-by: Bjorn Andersson <andersson@kernel.org>