sdhci_xenon: enable MMC FDT parsing

The mmc_fdt_parse allows to parse more MMC-related
FDT properties. Start using it. "wp-inverted" property,
VQMMC and newly added VMMC power supply parsing
is now done in a generic code.

Reviewed by: manu
Obtained from: Semihalf
Sponsored by: Marvell
Differential Revision: https://reviews.freebsd.org/D30562
MFC after: 2 weeks