Documentation: Update Phonet doc for Pipe Controller implementation

Updates the Phonet document with description related to Pipe controller
implementation

Signed-off-by: Kumar Sanghvi <kumar.sanghvi@stericsson.com>
Acked-by: Linus Walleij <linus.walleij@stericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>