soc/intel/common/systemagent: Add Kconfig guard

Looks like selecting SOC_INTEL_COMMON force-sets MMCONF_BASE_ADDR to
some value which can't be overriden outside of soc/intel/common. So
adding a non-SoC platform thats uses code from soc/intel/common is not
possible.

TEST=build test on wip platform

Change-Id: Ia160444e8ac7cac55153f659f4d98f4f77f0d467
Signed-off-by: Andrey Petrov <anpetrov@fb.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/38639
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Furquan Shaikh <furquan@google.com>
Reviewed-by: David Guckian
diff --git a/src/soc/intel/denverton_ns/Kconfig b/src/soc/intel/denverton_ns/Kconfig
index fa49eb0..aed2beb 100644
--- a/src/soc/intel/denverton_ns/Kconfig
+++ b/src/soc/intel/denverton_ns/Kconfig
@@ -54,6 +54,10 @@
 	select UDK_2015_BINDING
 	select CPU_INTEL_FIRMWARE_INTERFACE_TABLE
 
+config MMCONF_BASE_ADDRESS
+	hex
+	default 0xe0000000
+
 config FSP_T_ADDR
 	hex "Intel FSP-T (temp ram init) binary location"
 	depends on ADD_FSP_BINARIES && FSP_CAR