x86: Conditionally build the pinctrl_ich6 driver

The pinctrl_ich6 driver is currently unconditionally built for all
x86 boards. Let's use a Kconfig option to control the build.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 5c23b2c..18c7fb2 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -735,6 +735,12 @@
 	  slave) interrupt controllers. Include this to have U-Boot set up
 	  the interrupt correctly.
 
+config PINCTRL_ICH6
+	bool
+	help
+	  Intel ICH6 compatible chipset pinctrl driver. It needs to work
+	  together with the ICH6 compatible gpio driver.
+
 config I8254_TIMER
 	bool
 	default y