dt-bindings: hwmon: (adc128d818) Specify ti,mode property size

By default, cells in DT are 32-bit in size. The driver reads "ti,mode"
using the function of_property_read_u8() which causes the value to be
read incorrectly in little-endian architectures if the size is not
specified.

Make it explicit in the binding documentation that this prorperty must
be set as a 8-bit value.

Signed-off-by: Carlos Menin <menin@carlosaurelio.net>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>