dt-bindings: power: supply: bq25890: Document the auto_dpdm_disable option

The auto_dpdm_disable was added to disable dpdm negotiation so document it.
Signed-off-by: Angus Ainslie <angus@akkea.ca>

Documentation/devicetree/bindings/power/supply/bq25890.txt

@@ -33,6 +33,7 @@ Optional properties:
 - ti,thermal-regulation-threshold: integer, temperature above which the charge
     current is lowered, to avoid overheating (in degrees Celsius). If omitted,
     the default setting will be used (120 degrees);
+- ti,auto_dpdm_disable: disable the auto_dpdm negotiation
 
 Example: