Enable compilation of spi-bcm2835.c from the mainline on a Raspbian Kernel

Patch to make it possible to compile the mainline spi-bcm2835.c spi driver on the raspbian kernel, which still does not use device-tree as well as the 2708 Architecture name.

There are some advantages to this code:
* it uses the transfer one message interface and the corresponding "central" queuing mechanism
* the code is much cleaner.

What is still missing:
* the driver currently does not set the alternate functions for the PINs, so one has to load/unload spi-bcm2708 first to set them up
* there is no option to enable real time priorities to the work-queue
* the CS-Polarity bug has not been fixed either...

The last 2 are separate pull requests, so that they can get merged into the mainline as well...

I would also recommend enabling this driver as a module in the default compiles.

Signed-off-by: Martin Sperl <kernel@martin.sperl.org>