iio: Add channel for Gravity

Add new channel types support for gravity sensor.

Gravity sensor provides an application-level or physical collection that
identifies a device that measures exclusively the force of Earth's
gravity along any number of axes.

More information can be found in:
http://www.usb.org/developers/hidpage/HUTRR59_-_Usages_for_Wearables.pdf

Signed-off-by: Song Hongyan <hongyan.song@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>
diff --git a/tools/iio/iio_event_monitor.c b/tools/iio/iio_event_monitor.c
index d9b7e0f..b61245e 100644
--- a/tools/iio/iio_event_monitor.c
+++ b/tools/iio/iio_event_monitor.c
@@ -57,6 +57,7 @@
 	[IIO_RESISTANCE] = "resistance",
 	[IIO_PH] = "ph",
 	[IIO_UVINDEX] = "uvindex",
+	[IIO_GRAVITY] = "gravity",
 };
 
 static const char * const iio_ev_type_text[] = {
@@ -149,6 +150,7 @@
 	case IIO_RESISTANCE:
 	case IIO_PH:
 	case IIO_UVINDEX:
+	case IIO_GRAVITY:
 		break;
 	default:
 		return false;