pinctrl: Turn AMD support to tristate

The pinctrl-amd driver builds just fine as a module so give
users this option.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Cc: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>