iio: dac: ad5791: Add support for controlling RBUF via devicetree

This patch adds support for an external amplifier to be connected in a
gain of two configuration.
Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Nuno Sa <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20231129-ad5791-michael-stuff-v3-2-48e192b00909@analog.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/dac/ad5791.c

@@ -345,6 +345,7 @@ static int ad5791_probe(struct spi_device *spi)
 	struct iio_dev *indio_dev;
 	struct ad5791_state *st;
 	int ret, pos_voltage_uv = 0, neg_voltage_uv = 0;
+	bool use_rbuf_gain2;
 
 	indio_dev = devm_iio_device_alloc(&spi->dev, sizeof(*st));
 	if (!indio_dev)
@@ -379,6 +380,12 @@ static int ad5791_probe(struct spi_device *spi)
 	st->pwr_down = true;
 	st->spi = spi;
 
+	if (pdata)
+		use_rbuf_gain2 = pdata->use_rbuf_gain2;
+	else
+		use_rbuf_gain2 = device_property_read_bool(&spi->dev,
+							   "adi,rbuf-gain2-en");
+
 	if (!IS_ERR(st->reg_vss) && !IS_ERR(st->reg_vdd)) {
 		st->vref_mv = (pos_voltage_uv + neg_voltage_uv) / 1000;
 		st->vref_neg_mv = neg_voltage_uv / 1000;
@@ -398,7 +405,7 @@ static int ad5791_probe(struct spi_device *spi)
 
 
 	st->ctrl = AD5761_CTRL_LINCOMP(st->chip_info->get_lin_comp(st->vref_mv))
-		  | ((pdata && pdata->use_rbuf_gain2) ? 0 : AD5791_CTRL_RBUF) |
+		  | (use_rbuf_gain2 ? 0 : AD5791_CTRL_RBUF) |
 		  AD5791_CTRL_BIN2SC;
 
 	ret = ad5791_spi_write(st, AD5791_ADDR_CTRL, st->ctrl |