pinctrl: dt-bindings: samsung: Add header with values used for configuration

Hard-coded pinctrl configuration values are scattered through DTS files.
The numbers are difficult to decode by human, especially without the
datasheet.  Additionally the drive strength differs between S3C64xx,
S5PV210 and Exynos SoC families increasing the confusion.

The header will help making this more readable and maintainable.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Reviewed-by: Javier Martinez Canillas <javier@osg.samsung.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>