pinctrl: change Kconfig PINCTRL variable to a menuconfig

This allows PINCTRL to be selected manually to allow enabling of the
mcp23s08 i2c/spi gpio driver. Which is not platform specific.

Signed-off-by: Phil Reid <preid@electromag.com.au>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>