power: bq24735-charger: document 'ti,external-control' option

Implement an 'external-control' option for when the charger
shouldn't be configured by the kernel.
Signed-off-by: Jonathan Tinkham <sctincman@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sebastian Reichel <sre@kernel.org>

Documentation/devicetree/bindings/power_supply/ti,bq24735.txt

@@ -22,6 +22,9 @@ Optional properties :
    value must be between 128mA and 8.064A with a 128mA step resolution. The
    POR value is 0x1000h. This number is in mA (e.g. 8064), see the spec for
    more information about the InputCurrent (0x3fh) register.
+ - ti,external-control : Indicates that the charger is configured externally
+   and that the host should not attempt to enable/disable charging or set the
+   charge voltage/current.
 
 Example: