SPI: opmenu special case for WREN as atomic prefix

The code that attempts to use the opmenu needs to have a special
case for write enable now that it is handled as an atomic prefix
and not as a standalone opcode.

To test, ensure that runtime SPI write via ELOG is successful by
checking the event log for a kernel shutdown reason code:

5 | 2012-08-27 11:09:48 | Kernel Event | Clean Shutdown
6 | 2012-08-27 11:09:50 | System boot | 26
7 | 2012-08-27 11:09:50 | System Reset

Change-Id: I527638ef3e2a5ab100192c5be6e6b3b40916295a
Signed-off-by: Duncan Laurie <dlaurie@chromium.org>
Reviewed-on: http://review.coreboot.org/1710
Reviewed-by: Marc Jones <marcj303@gmail.com>
Tested-by: build bot (Jenkins)

src/include/spi.h

@@ -44,6 +44,9 @@
 #define SPI_XFER_BEGIN	0x01			/* Assert CS before transfer */
 #define SPI_XFER_END	0x02			/* Deassert CS after transfer */
 
+/* SPI opcodes */
+#define SPI_OPCODE_WREN 0x06
+
 /*-----------------------------------------------------------------------
  * Representation of a SPI slave, i.e. what we're communicating with.
  *