SB800: Add IMC ROM and fan control.

Add configuration for AMD's IMC ROM and fan registers for cimx/sb800
platforms.

- Allows user to add the IMC rom to the build and to configure the
  location of the "signature" between the allowed positions.
- Allows for no fan control, manual setup of SB800 Fan registers, or
  setup of the IMC fan configuration registers.
- Register configuration is done through devicetree.cb. No files need
  to be added for new platform configuration.
- Initial setup is for Persimmon, but may be extended to any cimx/sb800
  platform.

Change-Id: Ib06408d794988cbb29eed6adbeeadea8b2629bae
Signed-off-by: Martin Roth <martin@se-eng.com>
Reviewed-on: http://review.coreboot.org/1977
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-by: Marc Jones <marcj303@gmail.com>

src/vendorcode/amd/cimx/sb800/OEM.h

@@ -293,3 +293,13 @@
 #ifndef Spread_Spectrum_Type
   #define Spread_Spectrum_Type         0x00
 #endif
+
+/**
+ *    Imc Enable OverWrite
+ *    2 - by default strapping
+ *    1 - On
+ *    0 - Off
+ */
+#ifndef IMC_ENABLE_OVER_WRITE
+  #define IMC_ENABLE_OVER_WRITE        0x02
+#endif