dt-bindings: gpio: add gpio-line-names to rockchip,gpio-bank.yaml

Some people and companies may want to add more description
to there gpio pins. Add a gpio-line-names property to the
rockchip,gpio-bank.yaml file to reduce the notifications
from the existing mainline DT.

Signed-off-by: Johan Jonker <jbx6244@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>