dt-bindings: iio: adc: mcp3911: add reset-gpios

The MCP391X family provides an active low reset signal that is still not
described in the bindings.

Add reset-gpios to the bindings and the example.

Co-developed-by: Lukas Rauber <lukas.rauber@janitza.de>
Signed-off-by: Lukas Rauber <lukas.rauber@janitza.de>
Signed-off-by: Marcus Folkesson <marcus.folkesson@gmail.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://patch.msgid.link/20250428-mcp3911-fixes-v2-2-406e39330c3d@gmail.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>