release_notes: Update section about Counter driver

Add information about changes in counter driver.

Signed-off-by: Krzysztof Chruscinski <[email protected]>

Author: nordic-krch

doc/releases/release-notes-3.1.rst

@@ -422,6 +422,8 @@ Drivers and Sensors
 
 * Counter
 
+  * Added driver for NXP QTMR.
+
 * DAC
 
   *  support for ST STM32F1 to the ST STM32 DAC driver.