cpu/amd/microcode: Introduce CBFS access spinlock to avoid IOMMU failure

When microcode updates are enabled, this fixes an issue identical
to that described in GIT hash 7b22d84d:
 * drivers/pc80: Add optional spinlock for nvram CBFS access

Change-Id: Ib7e8cb171f44833167053ca98a85cca23021dfba
Signed-off-by: Timothy Pearson <tpearson@raptorengineeringinc.com>
Reviewed-on: https://review.coreboot.org/12063
Tested-by: build bot (Jenkins)
Reviewed-by: Martin Roth <martinroth@google.com>

src/Kconfig

@@ -489,6 +489,10 @@ config HAVE_ROMSTAGE_NVRAM_CBFS_SPINLOCK
 	  SR565x, that cannot handle concurrent CBFS accesses from
 	  multiple APs during early startup.
 
+config HAVE_ROMSTAGE_MICROCODE_CBFS_SPINLOCK
+	bool
+	default n
+
 config HAVE_MONOTONIC_TIMER
 	def_bool n
 	help