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 <andre.przywara@arm.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>