dt-bindings: iio: adc: add bindings for mcp3911

MCP3911 is a dual channel Analog Front End (AFE) containing two
synchronous sampling delta-sigma Analog-to-Digital Converters (ADC).

Co-Developed-by: Kent Gustavsson <kent@minoris.se>
Signed-off-by: Kent Gustavsson <kent@minoris.se>
Signed-off-by: Marcus Folkesson <marcus.folkesson@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>