pinctrl: remove pxa pinctrl driver

pinctrl-pxa driver doesn't support well on DT mode. Now pinctrl-single
could support DT mode & pin configuration. Use pinctrl-single driver to
support MMP/PXA silicons instead.

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