gpio: clean up gpio-ranges documentation

This change makes documentation of the the gpio-ranges property shorter
and more succinct, more consistent with the style of the rest of the
document, and not mention Linux-specifics such as the API
pinctrl_request_gpio(); DT binding documents should be OS independant
where at all possible. As part of this, the gpio-ranges property's format
is described in BNF form, in order to match the rest of the document.

This change also deprecates the #gpio-range-cells property. Such
properties are useful when one node references a second node, and that
second node dictates the format of the reference. However, that is not
the case here; the definition of gpio-ranges itself always dictates its
format entirely, and hence the value #gpio-range-cells must always be 3,
and hence there is no point requiring any referenced node to include
this property. The only remaining need for this property is to ensure
compatibility of DTs with older SW that was written to support the
previous version of the binding.

v4:
* Mention #gpio-range-cells as being deprecated, rather than removing all
  documentation of that property. This allows DTs to be written in a
  backwards-compatible way if desired, and also allows older DTs to be
  interpreted fully using the latest documentation.
v3:
* Mention BNF in commit description.
* Fixed typo.
* Dropped patch that removed the deprecated property from *.dts, since
  it's required to boot older kernels.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>