Documentation: Add examples to samsung-pinctrl device tree bindings documentation

This patch adds examples to samsung-pinctrl.txt documentaion file
on how to make gpio binding and gpio request

Signed-off-by: Leela Krishna Amudala <l.krishna@samsung.com>
Reviewed-by: Doug Anderson <dianders@chromium.org>
Reviewed-by: Tomasz Figa <t.figa@samsung.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>