pinctrl: pxa: export pxa2xx_pinctrl_init()

Building pinctrl-pxa27x.c as a module causes a link error:

ERROR: "pxa2xx_pinctrl_init" [drivers/pinctrl/pxa/pinctrl-pxa27x.ko] undefined!

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>