gpio: pxa: set initcall level to module init

gpio & pinctrl driver are used together. The pinctrl driver is already
launched before gpio driver in Makefile. So set gpio driver to module
init level. Otherwise, the sequence will be inverted.

Signed-off-by: Haojian Zhuang <haojian.zhuang@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>