pinctrl: Proofreading and updating the documentation accordingly

Proofreading and updating the documentation accordingly, i.e. fixed:

  - ambiguity of foo_set_mux() implementation and explanations
  - semantics in some of the examples, e.g. _probe() --> _init()
  - references to the callbacks to make them start with dot
  - references to the legacy API by replacing them with newer one
  - indentation in some of the examples
  - double words or phrases

updated:

  - unsigned --> unsigned int in some of the examples
  - use struct pingroup and PINCTRL_PINGROUP() in some of the examples
  - use struct pinfunction and PINCTRL_PINFUNCTION() in some of the examples

and enabled:

  - syntax highlighting for the examples in the programming languages
  - chapter references

Yet to clarify:

  - "gpioN" menton for the default function when requesting GPIO
Co-developed-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>