mfd: AXP20x: Add bindings documentation

Bindings documentation for the AXP20x driver. In this file also
sub-nodes are documented.

Signed-off-by: Carlo Caione <[e-mail]>
[[e-mail]: clarify interrupt source for the axp PMIC]
[[e-mail]: explain dcdc-workmode in detail and trim lines to 80 chars]
[[e-mail]: make regulator supplies optional if using unregulated input]
[[e-mail]: use cubieboard2 regulator nodes as example]
[[e-mail]: x-powers,dcdc-workmode default changed to 'current hardware setting']
[[e-mail]: reorganized regulator related properties into separate section.]
Signed-off-by: Chen-Yu Tsai <[e-mail]>
Acked-by: Maxime Ripard <[e-mail]>
Signed-off-by: Lee Jones <[e-mail]>

Documentation/devicetree/bindings/mfd/axp20x.txt

···
+AXP202/AXP209 device tree bindings
+
+The axp20x family current members :
+axp202 (X-Powers)
+axp209 (X-Powers)
+
+Required properties:
+- compatible: "x-powers,axp202" or "x-powers,axp209"
+- reg: The I2C slave address for the AXP chip
+- interrupt-parent: The parent interrupt controller
+- interrupts: SoC NMI / GPIO interrupt connected to the PMIC's IRQ pin
+- interrupt-controller: axp20x has its own internal IRQs
+- #interrupt-cells: Should be set to 1
+
+Optional properties:
+- x-powers,dcdc-freq: defines the work frequency of DC-DC in KHz
+		      (range: 750-1875). Default: 1.5MHz
+- <input>-supply: a phandle to the regulator supply node. May be omitted if
+		  inputs are unregulated, such as using the IPSOUT output
+		  from the PMIC.
+
+- regulators: A node that houses a sub-node for each regulator. Regulators
+	      not used but preferred to be managed by the OS should be
+	      listed as well.
+	      See Documentation/devicetree/bindings/regulator/regulator.txt
+	      for more information on standard regulator bindings.
+
+Optional properties for DCDC regulators:
+- x-powers,dcdc-workmode: 1 for PWM mode, 0 for AUTO (PWM/PFM) mode
+			  Default: Current hardware setting
+			  The DCDC regulators work in a mixed PWM/PFM mode,
+			  using PFM under light loads and switching to PWM
+			  for heavier loads. Forcing PWM mode trades efficiency
+			  under light loads for lower output noise. This
+			  probably makes sense for HiFi audio related
+			  applications that aren't battery constrained.
+
+
+AXP202/AXP209 regulators, type, and corresponding input supply names:
+
+Regulator	  Type		  Supply Name		  Notes
+---------	  ----		  -----------		  -----
+DCDC2		: DC-DC buck	: vin2-supply
+DCDC3		: DC-DC	buck	: vin3-supply
+LDO1		: LDO		: acin-supply		: always on
+LDO2		: LDO		: ldo24in-supply	: shared supply
+LDO3		: LDO		: ldo3in-supply
+LDO4		: LDO		: ldo24in-supply	: shared supply
+LDO5		: LDO		: ldo5in-supply
+
+Example:
+
+axp209: pmic@34 {
+	compatible = "x-powers,axp209";
+	reg = <0x34>;
+	interrupt-parent = <&nmi_intc>;
+	interrupts = <0 IRQ_TYPE_LEVEL_LOW>;
+	interrupt-controller;
+	#interrupt-cells = <1>;
+
+	regulators {
+		x-powers,dcdc-freq = <1500>;
+
+		vdd_cpu: dcdc2 {
+			regulator-always-on;
+			regulator-min-microvolt = <1000000>;
+			regulator-max-microvolt = <1450000>;
+			regulator-name = "vdd-cpu";
+		};
+
+		vdd_int_dll: dcdc3 {
+			regulator-always-on;
+			regulator-min-microvolt = <1000000>;
+			regulator-max-microvolt = <1400000>;
+			regulator-name = "vdd-int-dll";
+		};
+
+		vdd_rtc: ldo1 {
+			regulator-always-on;
+			regulator-min-microvolt = <1200000>;
+			regulator-max-microvolt = <1400000>;
+			regulator-name = "vdd-rtc";
+		};
+
+		avcc: ldo2 {
+			regulator-always-on;
+			regulator-min-microvolt = <2700000>;
+			regulator-max-microvolt = <3300000>;
+			regulator-name = "avcc";
+		};
+
+		ldo3 {
+			/* unused but preferred to be managed by OS */
+		};
+	};
+};