soc/intel/apollolake: clarify Fast SPI CS2 pad configuration

The pad for CS2 of the Fast SPI interface needs to be configured for
automatic MMIO translation when a SPI TPM is utilized. Instead of
unconditionally configuring that pad under LPC_TPM provide a explicit
Kconfig for a mainboard to select.

Change-Id: Ia94b90e12d71a4b849359188a853f7e036cc583b
Signed-off-by: Aaron Durbin <adurbin@chormium.org>
Reviewed-on: https://review.coreboot.org/14531
Reviewed-by: Andrey Petrov <andrey.petrov@intel.com>
Tested-by: build bot (Jenkins)

src/soc/intel/apollolake/Kconfig

@@ -41,6 +41,14 @@ config CPU_SPECIFIC_OPTIONS
 	select HAVE_HARD_RESET
 	select SOC_INTEL_COMMON
 
+config TPM_ON_FAST_SPI
+	bool
+	default n
+	select LPC_TPM
+	help
+	 TPM part is conntected on Fast SPI interface, but the LPC MMIO
+	 TPM transactions are decoded and serialized over the SPI interface.
+
 config SOC_INTEL_COMMON_RESET
 	bool
 	default y