sdhci_xenon: improve the VCCQ voltage switch sequence

Improve the VCCQ voltage switch, so that to properly
handle the SDHCI_HOST_CONTROL2 register signaling
flags and along with manipulating the regulator.

Reviewed by: manu
Obtained from: Semihalf
Sponsored by: Marvell
Differential Revision: https://reviews.freebsd.org/D30564
MFC after: 2 weeks