sdhci_xenon: allow to properly disable the UHS signaling

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 and not reported
in the bootverbose log.

Reviewed by: manu
Obtained from: Semihalf
Sponsored by: Marvell
Differential Revision: https://reviews.freebsd.org/D30563
MFC after: 2 weeks