pinctrl: add pinconf-generic define for a pin-default pull

There exist controllers that don't support to set the pull to up or down
separately but instead automatically set the pull direction based on
embedded knowledge inside the controller, for example depending on the
selected mux function of the pin.

Therefore this patch adds another config option to use this default
pull-state for a pin where it is not possible to know or decide if the
pin will be pulled up or down.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>