MFC: r273264, r274409, r278212, r278213:

MFC: r273264, r274409, r278212, r278213:

Add a workaround needed to fix a bug of Arasan Host Controller where it may
lose the contents of consecutive writes (that happens within two SD card
clock cycles).

This fixes the causes of instability during the SD card detection and
identification on Raspberry Pi (which happens at 400 kHz and so was much
more vulnerable to this issue).

Remove the previous workaround which clearly can't provide the same effect.

Remove stale comments about the issues with HS mode.

Remove a previous workaround to limit the minimum sdhci frequency that
isn't needed anymore.

Remove some duplicate calls to bus_release_resource() and destroy the mutex
on error cases.

While here remove unnecessary includes.

git-svn-id: svn://svn.freebsd.org/base/stable/10@278778 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f