gpio: mockup: implement naming the lines

In order to allow testing line lookup by name from user space, add
a new boolean parameter that indicates whether we want the lines to
be named. The name is created by concatenating the chip name and the
line offset value.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>