board/beaglebone/readme.txt: update list of DTB files produced

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

board/beaglebone/readme.txt

@@ -28,6 +28,10 @@ Result of the build
 ===================
 output/images/
 +-- am335x-boneblack.dtb
++-- am335x-boneblack-wireless.dtb
++-- am335x-boneblue.dtb
++-- am335x-bonegreen.dtb
++-- am335x-bonegreen-wireless.dtb
 +-- am335x-bone.dtb
 +-- am335x-evm.dtb
 +-- am335x-evmsk.dtb