iio: hid-sensor-accel-3d: Add timestamp channel for gravity sensor

The accel_3d sensor already has a timestamp channel, this patch just
replicate that for gravity sensor.
Signed-off-by: Ye Xiang <xiang.ye@intel.com>
Link: https://lore.kernel.org/r/20210105093515.19135-2-xiang.ye@intel.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/accel/hid-sensor-accel-3d.c

@@ -23,6 +23,7 @@ enum accel_3d_channel {
 	ACCEL_3D_CHANNEL_MAX,
 };
 
+#define CHANNEL_SCAN_INDEX_TIMESTAMP ACCEL_3D_CHANNEL_MAX
 struct accel_3d_state {
 	struct hid_sensor_hub_callbacks callbacks;
 	struct hid_sensor_common common_attributes;
@@ -75,7 +76,7 @@ static const struct iio_chan_spec accel_3d_channels[] = {
 		BIT(IIO_CHAN_INFO_HYSTERESIS),
 		.scan_index = CHANNEL_SCAN_INDEX_Z,
 	},
-	IIO_CHAN_SOFT_TIMESTAMP(3)
+	IIO_CHAN_SOFT_TIMESTAMP(CHANNEL_SCAN_INDEX_TIMESTAMP)
 };
 
 /* Channel definitions */
@@ -110,7 +111,8 @@ static const struct iio_chan_spec gravity_channels[] = {
 		BIT(IIO_CHAN_INFO_SAMP_FREQ) |
 		BIT(IIO_CHAN_INFO_HYSTERESIS),
 		.scan_index = CHANNEL_SCAN_INDEX_Z,
-	}
+	},
+	IIO_CHAN_SOFT_TIMESTAMP(CHANNEL_SCAN_INDEX_TIMESTAMP),
 };
 
 /* Adjust channel real bits based on report descriptor */