sdhci: Provide devmethod for software reset

Some sdhci controllers require custom software reset logic. Accommodate
this need by introducing a new SDHCI_RESET devmethod. Move the existing
reset logic into sdhci_generic_reset and use it as a default for the
aforementioned method.

Obtained from: Semihalf
Sponsored by: Alstom Group
Differeential revision: https://reviews.freebsd.org/D32704