cxl: Add CXL_KERNEL_API config option

Add CXL_KERNEL_API config option so drivers which depend on this new
functionality won't be enabled until this is visible.

This is useful for merging the cxlflash driver which comes in via the SCSI
tree.  The cxlflash driver can depend on CXL_KERNEL_API, hence it won't be
enabled in the SCSI tree until this new config option is merged via the powerpc
tree.  Hence all trees will be bisectable at all times.
Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

drivers/misc/cxl/Kconfig

@@ -7,10 +7,15 @@ config CXL_BASE
 	default n
 	select PPC_COPRO_BASE
 
+config CXL_KERNEL_API
+	bool
+	default n
+
 config CXL
 	tristate "Support for IBM Coherent Accelerators (CXL)"
 	depends on PPC_POWERNV && PCI_MSI
 	select CXL_BASE
+	select CXL_KERNEL_API
 	default m
 	help
 	  Select this option to enable driver support for IBM Coherent