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>