dt-bindings: mfd: Add Actions Semi ATC260x PMIC binding

Add devicetree binding for Actions Semi ATC260x PMICs.

Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>