Until now the "no-1-8-v" DT flag wrongly disabled the SDHCI_CAN_VDD_180
- slot 1.8V power supply capability, whereas it refers to the signaling
voltage. Fix the sdhci_xenon_read_4 and allow to disable the UHS modes
depending on the DT property or PHY slow mode. While at it - make sure
the unsupported 1.2V signaling is always disabled.