pinctrl: stm32: Fix compile testing selection

While selecting the driver for compile testing seemed possible,
the driver was not compiled because the driver directory was only
added if ARCH_STM32 was selected.

This patch now makes the pinctrl Makefile to add stm32 directory if
PINCTRL_STM32 is selected.

Signed-off-by: Maxime Coquelin <mcoquelin.stm32@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>