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>