iioservice: Add Max Type in cros::mojom::DeviceType

This commit adds the max type to help determine the maximum supported
device types.

BUG=b:162154663
TEST=builds

Change-Id: I56a06cacd55f80c6e9ee69a421625e6cffa2455f
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform2/+/2319426
Reviewed-by: Gwendal Grignou <gwendal@chromium.org>
Reviewed-by: Cheng-Hao Yang <chenghaoyang@chromium.org>
Commit-Queue: Cheng-Hao Yang <chenghaoyang@chromium.org>
Tested-by: Cheng-Hao Yang <chenghaoyang@chromium.org>
Auto-Submit: Cheng-Hao Yang <chenghaoyang@chromium.org>
diff --git a/iioservice/mojo/sensor.mojom b/iioservice/mojo/sensor.mojom
index 9e6d211..fe13bcf 100644
--- a/iioservice/mojo/sensor.mojom
+++ b/iioservice/mojo/sensor.mojom
@@ -16,6 +16,7 @@
   ANGL = 6,
   ACPI_ALS = 7,
   BARO = 8,
+  MAX = 9,
 };
 
 enum ObserverErrorType {