iio: ltc2983: add support for optional reset gpio

Check if an optional reset gpio is present and if so, make sure to reset
the device.

Signed-off-by: Nuno Sá <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20210825084149.11587-1-nuno.sa@analog.com


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