gpio: samsung: add flags specifier to device-tree binding

This adds a flags field to the gpio specifier for Samsung. I didn't
want to add yet another field in the already quite long specifier, so
I decided to compress it together with the Pull Up/Down settings instead.

This is needed to, for example, have a gpio-keys input that is active low.

Signed-off-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>