gpio: mockup: add set_config to support pull up/down

Add support for the pull up/down state set via gpiolib line requests to
be reflected in the state of the mockup.
Use case is for testing of the GPIO uAPI, specifically the pull up/down
flags.

Signed-off-by: Kent Gibson <warthog618@gmail.com>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>