mfd: lochnagar: Add links to binding docs for sound and hwmon

Lochnagar is an evaluation and development board for Cirrus
Logic Smart CODEC and Amp devices. It allows the connection of
most Cirrus Logic devices on mini-cards, as well as allowing
connection of various application processor systems to provide a
full evaluation platform. This driver supports the board
controller chip on the Lochnagar board.

Add links to the binding documents for the new sound and hardware
monitor parts of the driver.

Signed-off-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>