dt-bindings: gnss: Convert UBlox Neo-6M binding to a schema

The UBlox Neo 6M is supported by Linux thanks to its device tree
binding.

Now that we have the DT validation in place, let's convert the device
tree bindings for that driver over to a YAML schema.

Cc: Johan Hovold <johan@kernel.org>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://lore.kernel.org/r/20210901091852.479202-22-maxime@cerno.tech
Signed-off-by: Rob Herring <robh@kernel.org>