ASoC: cs35l35: Add device tree binding for I2S drive configuration

Add a binding for setting how the I2S pins are driven in unused slots,
currently the chip will just use the default of drive 0, however
this causes issues when multiple devices are attached to the same bus.

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

@@ -37,6 +37,11 @@ Optional properties:
   Table 3-10 of the datasheet lists drive-strength specifications
   0 = 1x (Default)
   1 = .5x
+  - cirrus,sp-drv-unused : Determines how unused slots should be driven on the
+  Serial Port.
+  0 - Hi-Z
+  2 - Drive 0's (Default)
+  3 - Drive 1's
 
   - cirrus,bst-pdn-fet-on : Boolean to determine if the Boost PDN control
   powers down with a rectification FET On or Off. If VSPK is supplied