dt-bindings: ARM: add bindings for the Meraki MR26

mark the Meraki MR26 and its SoC (BCM53015) as supported
in the bindings documentation file.

<https://jp.broadcom.com/products/embedded-and-networking-processors/communications/bcm5301x>

"BCM53011 and BCM53015 with dual-core Cortex-A9 CPU at 800 MHz,
256KB L2 cache, 16-bit DDR2 interface, USB3, integrated switch,
GPHYs and packet accelerator"
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>

Documentation/devicetree/bindings/arm/bcm/brcm,bcm4708.yaml

@@ -87,6 +87,13 @@ properties:
           - const: brcm,bcm53012
           - const: brcm,bcm4708
 
+      - description: BCM53015 based boards
+        items:
+          - enum:
+              - meraki,mr26
+          - const: brcm,bcm53015
+          - const: brcm,bcm4708
+
       - description: BCM53016 based boards
         items:
           - enum: