sdhci: allow setting MMC capabilities before sdhci_init_slot

With this change the host controller drivers can set the MMC capabilities
(e.g. using mmc_fdt_parse() helper) before calling sdhci_init_slot().
This way the configuration dump (eg. in bootverbose) can include the
possible additional information.

Reviewed by: manu
Obtained from: Semihalf
Sponsored by: Marvell
Differential Revision: https://reviews.freebsd.org/D30561
MFC after: 2 weeks