Configure GPIOs for I2S based on revision/card settings

With RPi model B+, assignment of the I2S GPIO pins has changed.
This patch uses the board revision to auto-detect the GPIOs used
for I2S. It also allows sound card drivers to set the GPIOs that
should be used. This is especially important with the Compute
Module.
Merge pull request #648 from hifiberry/rpi-3.12.y

Configure GPIOs for I2S based on revision/card settings