doc: updated doc for Zephyr release 1.9.

Updated doc for release 1.9 for :-

1. Added device tree support for Intel Quark based microcontroller
   boards.

Signed-off-by: Savinay Dharmappa <[email protected]>

Author: SavinayDharmappa

doc/release-notes-1.9.rst

@@ -43,7 +43,8 @@ Architectures
 Boards
 ******
 
-* change description
+* Added device tree support for Intel Quark based microcontroller boards
+  such as Arduino_101, tinytile, and Quark_d2000_crb.
 
 Drivers and Sensors
 *******************