mfd: dt: Add ranges, #address-cells and #size-cells as optional properties

Whilst describing a device and not a bus, simple-mfd is modelled on
simple-bus where child nodes are iterated and registered as platform
devices. Some complex devices, e.g. the Aspeed LPC controller, can
benefit from address space mapping such that child nodes can use the
regs property to describe their resource offsets within the
multi-function device.

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>