ASoC: cs35l35: Add DT binding to specify usage of an external boost supply

Add a device tree binding to let the driver know that the amplifier is
configured to use an external boost supply.
Signed-off-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Acked-by: Brian Austin <brian.austin@cirrus.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/cs35l35.txt

@@ -33,6 +33,10 @@ Optional properties:
   - cirrus,shared-boost : Boolean to enable ClassH tracking of Advisory Signal
   if 2 Devices share Boost BST_CTL
 
+  - cirrus,external-boost : Boolean to specify the device is using an external
+  boost supply, note that sharing a boost from another cs35l35 would constitute
+  using an external supply for the slave device
+
   - cirrus,sp-drv-strength : Value for setting the Serial Port drive strength
   Table 3-10 of the datasheet lists drive-strength specifications
   0 = 1x (Default)