gpio: of: optimize "gpios" property parsing of of_parse_own_gpio()

Call of_property_read_u32_array() only once rather than iterating
of_property_read_u32_index().

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