gpiolib: add support for bias pull disable

This change prepares the gpio core to look at firmware flags and set
'FLAG_BIAS_DISABLE' if necessary. It works in similar way to
'GPIO_PULL_DOWN' and 'GPIO_PULL_UP'.

Signed-off-by: Nuno Sá <nuno.sa@analog.com>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>