intel/sandybridge: Fix builds with System Agent blob

Broken with commit:
  5c10abe nb/intel/sandybridge: increase MMCONF_BASE_ADDRESS

Available sandybridge/systemagent-r6.bin has MMCONF hard-coded
at some places and samsung/lumpy fails at boot here:

  CBFS: Locating 'mrc.bin'
  CBFS: Found @ offset 9fec0 size 2fc94
  System Agent: Starting up...
  System Agent: Initializing

These are the last lines as captured over USB debug.

Change-Id: I441847f0e71a5e1be9c8ef6a04a81eb7bdd8a6d9
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: https://review.coreboot.org/15328
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Rudolph <siro@das-labor.org>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

src/northbridge/intel/sandybridge/Kconfig

@@ -94,4 +94,8 @@ config MRC_FILE
 	  The path and filename of the file to use as System Agent
 	  binary.
 
+config MMCONF_BASE_ADDRESS
+	hex
+	default 0xf0000000 if !USE_NATIVE_RAMINIT
+
 endif