gpio: OF: Cut painful BNF experiments from bindings

In 2011 the commit bf859f84
("gpio/dt: Refine GPIO device tree binding") introduced an
experimental BNF notation for defining a regular grammar for
the GPIO phandles used by different devices.

This was an interesting approach, and shows that we have long
nutured the idea to formally verify device tree files using
regular grammar.

Most if not all other bindings use natural language to define
the bindings, and the recent thinking for verifying device
tree files is to use JSON schemas in separate definitions.

Cut the BNF business and replace it with natural language
so that it becomes more human-readable for now.

Cc: devicetree@vger.kernel.org
Cc: Grant Likely <grant.likely@secretlab.ca>
Cc: Kumar Gala <galak@kernel.crashing.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>