tps65090: Update bindings for subnode

This updates the DT documentation to reflect that the tps65090-charger
should be represented as a child node of the tps65090 device itself.
Signed-off-by: Rhyland Klein <rklein@nvidia.com>
Signed-off-by: Anton Vorontsov <anton@enomsg.org>

Documentation/devicetree/bindings/power_supply/tps65090.txt

 TPS65090 Frontend PMU with Switchmode Charger
 
 Required Properties:
--compatible: "ti,tps65090"
--reg: I2C slave address
--interrupts: the interrupt output to which this device connects
+-compatible: "ti,tps65090-charger"
 
 Optional Properties:
 -ti,enable-low-current-chrg: Enables charging when a low current is detected
  while the default logic is to stop charging.
 
-Example:
+This node is a subnode of the tps65090 PMIC.
 
-	tps65090@48 {
-		compatible = "ti,tps65090";
-		reg = <0x48>;
-		interrupts = <0 88 0x4>;
+Example:
 
+	tps65090-charger {
+		compatible = "ti,tps65090-charger";
 		ti,enable-low-current-chrg;
-
-		regulators {
-			...
-		};
+	};