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/ivybridge/Kconfig b/arch/x86/cpu/ivybridge/Kconfig
index eec92df..5f0e608 100644
--- a/arch/x86/cpu/ivybridge/Kconfig
+++ b/arch/x86/cpu/ivybridge/Kconfig
@@ -13,6 +13,7 @@
 	imply AHCI_PCI
 	imply ICH_SPI
 	imply INTEL_ICH6_GPIO
+	imply PINCTRL_ICH6
 	imply SCSI
 	imply SCSI_AHCI
 	imply SPI_FLASH