I2S patch for Raspberry Pi Model B+ and compute module

This patch of the I2S subsystem allows to configure the I2S subsystem to the correct pins. With Raspberry Pi model A and B they were on GPIO28+, at Model B+ they are at GPIO18+ and on the compute module they can be configured in both ways. This patch used auto-detection based on the board revision, but also provides a function to allow the sound card driver to select the correct GPIO.