gpio: sim: delimit the fwnode name with a ":" when generating labels

Typically, whenever a human-readable name is created for objects using
a software node, its name is delimited with ":" as dashes are often used
in other parts of the name. Make gpio-sim use the same pattern. This
results in better looking default names:

  gpio-sim.0:node0
  gpio-sim.0:node1
  gpio-sim.1:node0

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>