Added support for proto i2s DAC board

This is a reworking of (https://github.com/raspberrypi/linux/pull/480) to take account of comments received from koalo.  It implements the changes needed to get Mikroelektronika's Proto i2s DAC board (WM8731 based) working.  It is basically the 3.8 board driver with a couple of 3.10-related changes. 

As koalo indicated, I have split this into 2 commits, one for bcm2708.c and one for the other changed/inserted files.

Sorry for reissuing this as a new pull request, I over-committed on my original fork which made my changes difficult to see.

BBUK