SPI bitbanging becomes library code

This updates the SPI bitbanging support so it should be usable for some common types of SPI hardware.

This updates the SPI bitbanging support:

- It's no longer a standalone driver, but is instead a library of code for drivers to call.

- It should now support three different types of driver, all within this task-per-controller implementation model.

* Pure bitbang. These can now provide word-at-a-time I/O loops tuned for each SPI mode, possibly using assembly language.

* Word-at-a-time controllers. Some controllers essentially let those I/O loops be handled by the hardware, and at common CPU and SPI speeds a polling driver may be quite appropriate.

* Transfer-at-a-time controllers. It turns out that it's easy to fit controllers with FIFOs, half-duplex restrictions, and simple DMA support into the same framework.

- Some cleanups to the code actually banging bits.

That means a lot of hardware could work with this framework, though common types of controller can't reach peak performance without switching to a driver structure that supports pipelining of transfers (e.g. DMA queues) and maybe controllers (e.g. IRQ driven).

@@ -71,14 +71,14 @@ config SPI_BITBANG
 	depends on SPI_MASTER && EXPERIMENTAL
 	help
 	  With a few GPIO pins, your system can bitbang the SPI protocol.
-	  Choose this if you need SPI support through I/O pins (GPIO,
-	  parallel port, etc).
-
-	  Make sure you initialize one platform device object for each SPI
-	  master device on your system, using "spi_bitbang" as the device
-	  name with platform_data saying how to use which pins.  That will
-	  often be done using static board-specific tables in board-specific
-	  (or adapter-specific) setup code.
+	  Select this to get SPI support through I/O pins (GPIO, parallel
+	  port, etc).  Or, some systems' SPI master controller drivers use
+	  this code to manage the per-word or per-transfer accesses to the
+	  hardware shift registers.
+
+	  This is library code, and is automatically selected by drivers that
+	  need it.  You only need to select this explicitly to support driver
+	  modules that aren't part of this kernel tree.
 
 #
 # Add new SPI master controllers in alphabetical order above this line