iio: dac: ad5791: Add reset, clr and ldac gpios

iio: dac: ad5791: Add reset, clr and ldac gpios

The ad7591 has reset, clr and ldac gpios. For the DAC to output data
continuously written to the data register the state of these gpios needs
to be set by the driver.

Add these gpios to the driver making them optional in case they are fixed
on the pcb.

Reviewed-by: David Lechner <dlechner@baylibre.com>
Signed-off-by: Axel Haslam <ahaslam@baylibre.com>
Link: https://patch.msgid.link/20241031071746.848694-5-ahaslam@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>