dt-bindings: bus: Drop empty compatible string in example

In preparation to add generic checks of compatible strings, drop
the compatible as '...' is not a valid compatible string.

Cc: Maxime Ripard <mripard@kernel.org>
Cc: Chen-Yu Tsai <wens@csie.org>
Cc: linux-arm-kernel@lists.infradead.org
Signed-off-by: Rob Herring <robh@kernel.org>