iio: addac: ad74413r: add support for reset-gpio

iio: addac: ad74413r: add support for reset-gpio

We have a board where the reset pin of the ad74412 is connected to a
gpio, but also pulled low (i.e., asserted) by default. Hence to get
the chip out of reset, the driver needs to know about that gpio and
deassert the reset signal before attempting to communicate with the
chip.

When a reset-gpio is given in device tree, use that instead of the
software reset. According to the data sheet, the two methods are
functionally equivalent.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Link: https://lore.kernel.org/r/20221115095517.1008632-4-linux@rasmusvillemoes.dk
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>