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

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

Eliminate devclass_find_free_unit call here, since -1 gives the same
net behavior.

- Fix build with INVARIANTS enabled.

- Whitespace nitpicking.

- Add the driver for AT91RM9200 CompactFlash controller. The driver
operates in the common memory mode and use polling mode to control
the status of operations as I don't have any board with interrupt
line routed yet. I'll add the GPIO interrupt driven mode as soon
as I get one.