ASoC: cs35l35: Allow user to configure IMON SCALE

On the chip the IMON signal is a full 24-bits however normally only
some of the bits will be sent over the bus. The chip provides a field
to select which bits of the IMON will be sent back, this is the only
feedback signal that has this feature.

Add an additional entry to the cirrus,imon device tree property to
allow the IMON scale parameter to be passed.

Signed-off-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Acked-by: Brian Austin <brian.austin@cirrus.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>