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/cpu/baytrail/Kconfig b/arch/x86/cpu/baytrail/Kconfig
index ac58b03..022a9f2 100644
--- a/arch/x86/cpu/baytrail/Kconfig
+++ b/arch/x86/cpu/baytrail/Kconfig
@@ -12,6 +12,7 @@
 	imply AHCI_PCI
 	imply ICH_SPI
 	imply INTEL_ICH6_GPIO
+	imply PINCTRL_ICH6
 	imply MMC
 	imply MMC_PCI
 	imply MMC_SDHCI