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 <[e-mail]>
Cc: Linus Walleij <[e-mail]>
Signed-off-by: Linus Walleij <[e-mail]>

drivers/pinctrl/Kconfig

···
 	  controller available on sama5d2 SoC.
 
 config PINCTRL_AMD
-	bool "AMD GPIO pin control"
+	tristate "AMD GPIO pin control"
 	depends on GPIOLIB
 	select GPIOLIB_IRQCHIP
 	select PINCONF