doc: release-notes-3.1: add release notes for USB

Add release notes for USB.

Signed-off-by: Johann Fischer <[email protected]>

Author: jfischer-no

doc/releases/release-notes-3.1.rst

@@ -575,7 +575,7 @@ Drivers and Sensors
 
 * USB
 
-  * Raspberry Pi Pico: Added USB driver
+  * Added RP2040 (Raspberry Pi Pico) USB device controller driver
 
 * Watchdog
 
@@ -695,6 +695,9 @@ Networking
 USB
 ***
 
+* Moved USB device stack code to own directory in preparation for upcoming
+  rework of USB support.
+
 Build System
 ************