ARM: imx: select PINCTRL at sub-architecure level

Instead of selecting PINCTRL on individual SoC, let's select it at IMX
sub-architecure level.

While at it, it also adds the missing PINCTRL_IMX50 selection for
SOC_IMX50.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>