Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Remove contractions.

Add support for RICOH R5CE823 card reader, that can be found in
some Lenovo laptops.

The conroller needs a quirk to lower its frequency, and after
that it operates normally.

Add some information in SYNOPSIS, HARDWARE and BUGS sections.

Add sdhci driver man page. Link it up to mmc subsystem.