gpio: Rework of_gpiochip_set_names() to use device property accessors

In order to use "gpio-line-names" property in systems not having DT as
their boot firmware, rework of_gpiochip_set_names() to use device property
accessors. This reworked function is placed in a separate file making it
clear it deals with universal device properties.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>