mmc_spi host driver

This is the latest version of the MMC-over-SPI support.  It works
on 2.6.23-rc2 plus git-mmc (from rc1-mm2), along with the preceding
patches which teach the rest of the MMC stack about SPI.

The main issue of note is that sometimes cards need to be power cycled
to recover after certain faults.  Also, it may sometimes be necessary
to disable CRCs.  ("modprobe mmc_core use_spi_crc=n")

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Cc: mikael.starvik@axis.com,
Cc: Hans-Peter Nilsson <hp@axis.com>
Cc: Jan Nikitenko <jan.nikitenko@gmail.com>
Cc: Mike Lavender <mike@steroidmicros.com>
Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>