gpio: of: factor out common code to a new helper function

The conversion from a DT spec to struct gpio_desc is common between
of_get_named_gpiod_flags() and of_parse_own_gpio().  Factor out the
common code to a new helper, of_xlate_and_get_gpiod_flags().

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>