dt-bindings: mfd: axp20x: Add fallback for axp805

axp805 is actually compatible and used with axp806 as fallback.
But it's actually undocumented and trig a warning with checkpatch.

DT compatible string "x-powers,axp805" appears un-documented.

Add this compatible in the dt-bindings documentation.
Signed-off-by: Clément Péron <peron.clem@gmail.com>
Signed-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/mfd/axp20x.txt

@@ -25,6 +25,7 @@ Required properties:
     * "x-powers,axp223"
     * "x-powers,axp803"
     * "x-powers,axp806"
+    * "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