mfd: arizona: Update binding docs for selecting mono/stereo outputs

Update the device tree binding documentation to include the wlf,out-mono
property that is used to specify whether each output is a mono or stereo
output.

Signed-off-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>