dt-bindings: hwmon: ina2xx: Describe #io-channel-cells property

There are two drivers in the Linux kernel. One is hwmon based and second
IIO. IIO version requires to define #io-channel-cells to operate.

Signed-off-by: Michal Simek <michal.simek@amd.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/aa303b9fe3116e7f98d6b72822f7f57694366db3.1703077926.git.michal.simek@amd.com


Signed-off-by: Guenter Roeck <linux@roeck-us.net>