Added autodetection (based on board revision) and driver based setting of I2S GPIO pins.

GPIOs used for I2S are different on Model A/B and B+ and undefined on the compute module.
This patch adds autodetected based on the board revision. It also adds a function to
set GPIO pins used for I2S. This allows a sound card driver to set the correct GPIOs.
If this is not configured by the sound card driver, autodetection is used based.
Merge branch 'rpi-3.12.y' of https://github.com/hifiberry/linux into rpi-3.12.y