pinctrl: pinconf-generic: add generic APIs for mapping pinctrl node

Add generic APIs to map the DT node and its sub node in pinconf generic
driver. These APIs can be used from driver to parse the DT node who
uses the pinconf generic APIs for defining their nodes.

Changes from V1:
- Add generic property for pins and functions in pinconf-generic.
- Add APIs to map the DT and subnode.
- Move common utils APIs to the pinctrl-utils from this file.
- Update the binding document accordingly.
Changes from V2:
- Rebased the pinctrl binding doc on top of Stephen's cleanup.
- Rename properties "pinctrl-pins" and "pinctrl-function" to
  "pins" and "function".

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Reviewed-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>