dt-bindings: mfd: axp20x: Add AXP305 compatible (plus optional IRQ)

The AXP305 PMIC used on many boards with the H616 SoC seems to be fully
compatible to the AXP805 PMIC, so add the proper chain of compatible
strings.

Also at least on one board (Orangepi Zero2) there is no interrupt line
connected to the CPU, so make the "interrupts" property optional.

Signed-off-by: Andre Przywara <[e-mail]>
Acked-by: Rob Herring <[e-mail]>
Signed-off-by: Lee Jones <[e-mail]>

Documentation/devicetree/bindings/mfd/axp20x.txt

···
     * "x-powers,axp803"
     * "x-powers,axp806"
     * "x-powers,axp805", "x-powers,axp806"
+    * "x-powers,axp305", "x-powers,axp805", "x-powers,axp806"
     * "x-powers,axp809"
     * "x-powers,axp813"
 - reg: The I2C slave address or RSB hardware address for the AXP chip
-- interrupts: SoC NMI / GPIO interrupt connected to the PMIC's IRQ pin
 - interrupt-controller: The PMIC has its own internal IRQs
 - #interrupt-cells: Should be set to 1
 
···
 			AXP20x/LDO3: software-based implementation
 
 Optional properties:
+- interrupts: SoC NMI / GPIO interrupt connected to the PMIC's IRQ pin
 - x-powers,dcdc-freq: defines the work frequency of DC-DC in KHz
 		      AXP152/20X: range:  750-1875, Default: 1.5 MHz
 		      AXP22X/8XX: range: 1800-4050, Default: 3   MHz