Documentation: gpio: Replace leading TABs by spaces in code blocks

Code blocks are indented with two spaces. If there's a leading TAB, the
syntax highlighting might be broken in some editors. To prevent that,
unify all code blocks by using spaces instead of leading TAB(s).

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
[Bartosz: tweaked the commit message]
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>