sdhci_xenon: add UHS support

This patch adds the necessary methods resolution to the sdhci_xenon
driver which are required to configure UHS modes for SD/MMC devices.
Apart from the two generic routines, the custom sdhci_xenon_set_uhs_timing
function is responsible for setting the SDHCI_HOST_CONTROL2 register
with appropriate mode select values - in case of HS200 and HS400
they are non-standard.

Reviewed by: manu
Obtained from: Semihalf
Sponsored by: Marvell
Differential Revision: https://reviews.freebsd.org/D30565
MFC after: 2 weeks