dt-bindings: arm: bcm: Add a select to the RPI Firmware binding

The RaspberryPi firmware binding uses two compatible, include simple-bus.
The select statement generated by default will thus select any node that
has simple-bus, not all of them being the raspberrypi firmware node.

This results in warnings being wrongfully reported. Let's add a custom
select statement to fix that.

Fixes: d4c708c0 ("dt-bindings: arm: bcm: Convert BCM2835 firmware binding to YAML")
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://lore.kernel.org/r/20200626115433.125735-1-maxime@cerno.techAcked-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>

Documentation/devicetree/bindings/arm/bcm/raspberrypi,bcm2835-firmware.yaml

@@ -10,6 +10,15 @@ maintainers:
   - Eric Anholt <eric@anholt.net>
   - Stefan Wahren <wahrenst@gmx.net>
 
+select:
+  properties:
+    compatible:
+      contains:
+        const: raspberrypi,bcm2835-firmware
+
+  required:
+    - compatible
+
 properties:
   compatible:
     items: