iio: adc: xilinx: Fix VREFP scale

The scaling factor for VREFP is 3.0/4096, not 1.0/4096; fix this to get
correct readings.
Signed-off-by: NThomas Betker <thomas.betker@rohde-schwarz.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: NJonathan Cameron <jic23@kernel.org>

drivers/iio/adc/xilinx-xadc-core.c

@@ -856,6 +856,7 @@ static int xadc_read_raw(struct iio_dev *indio_dev,
 			switch (chan->address) {
 			case XADC_REG_VCCINT:
 			case XADC_REG_VCCAUX:
+			case XADC_REG_VREFP:
 			case XADC_REG_VCCBRAM:
 			case XADC_REG_VCCPINT:
 			case XADC_REG_VCCPAUX: