uinput: document the port field for device registration

Bug: 245989146
Change-Id: I84dfce5d216a5065611bf63f61e803a93e76b58f
Test: check gittiles rendering

cmds/uinput/README.md

@@ -48,6 +48,7 @@ Register a new uinput device
 | `vid`            | 16-bit integer | Vendor ID                  |
 | `pid`            | 16-bit integer | Product ID                 |
 | `bus`            | string         | Bus that device should use |
+| `port`           | string         | `phys` value to report     |
 | `configuration`  | object array   | uinput device configuration|
 | `ff_effects_max` | integer        | `ff_effects_max` value     |
 | `abs_info`       | array          | Absolute axes information  |