pinctrl: pxa: guard sub-directory with CONFIG_PINCTRL_PXA

CONFIG_PINCTRL_PXA is more suitable than CONFIG_ARCH_PXA
to guard the drivers/pinctrl/pxa/ directory.

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