dt-bindings: gpio: add pull-disable flag

This extends the flags that can be used in GPIO specifiers to indicate
that no bias is intended in the pin.

Signed-off-by: Nuno Sá <nuno.sa@analog.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>