DT: bindings: bcm: Add Raspberry Pi Zero

Signed-off-by: Stefan Wahren <[e-mail]>
Acked-by: Rob Herring <[e-mail]>
Acked-by: Stephen Warren <[e-mail]>
Signed-off-by: Eric Anholt <[e-mail]>

Documentation/devicetree/bindings/arm/bcm/brcm,bcm2835.txt

···
 Required root node properties:
 compatible = "raspberrypi,compute-module", "brcm,bcm2835";
 
+Raspberry Pi Zero
+Required root node properties:
+compatible = "raspberrypi,model-zero", "brcm,bcm2835";
+
 Generic BCM2835 board
 Required root node properties:
 compatible = "brcm,bcm2835";