board/raspberrypi/readme.txt: add missing dtb files

Add missing dtb files:
 - bcm2708-rpi-cm.dtb from genimage-raspberrypi.cfg
 - bcm2708-rpi-zero.dtb from genimage-raspberrypi0.cfg:
 - bcm2708-rpi-zero-w.dtb from genimage-raspberrypi0w.cfg
 - bcm2710-rpi-cm3.dtb from genimage-raspberrypi3.cfg
 - bcm2837-rpi-3-b.dtb from genimage-raspberrypi3-64.cfg

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

board/raspberrypi/readme.txt

@@ -76,11 +76,16 @@ After building, you should obtain this tree:
     output/images/
     +-- bcm2708-rpi-b.dtb           [1]
     +-- bcm2708-rpi-b-plus.dtb      [1]
+    +-- bcm2708-rpi-cm.dtb          [1]
+    +-- bcm2708-rpi-zero.dtb        [1]
+    +-- bcm2708-rpi-zero-w.dtb      [1]
     +-- bcm2709-rpi-2-b.dtb         [1]
     +-- bcm2710-rpi-3-b.dtb         [1]
     +-- bcm2710-rpi-3-b-plus.dtb    [1]
+    +-- bcm2710-rpi-cm3.dtb         [1]
     +-- bcm2711-rpi-4-b.dtb         [1]
     +-- bcm2711-rpi-cm4.dtb         [1]
+    +-- bcm2837-rpi-3-b.dtb         [1]
     +-- boot.vfat
     +-- rootfs.ext4
     +-- rpi-firmware/