gpiolib: of: improve gpiolib-of support of pull up/down on expanders

When using GPIO expanders attached to I2C ports, their set_config function
needs to be passed a config setting which contains options to enable pull
up or pull down bias feature.  In order to set this config properly,
the gpio parser needs to handle GPIO_PULL_UP and GPIO_PULL_DOWN.

This patch enables the flags corresponding to GPIO_PULL_UP and
GPIO_PULL_DOWN.

Signed-off-by: Adam Ford <aford173@gmail.com>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>