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

(cherry picked from commit c80e2ca57e0c1b3647b55471584c6d32214232ea)