cxl: Export file ops for use by API

The cxl kernel API will allow drivers other than cxl to export a file
descriptor which has the same userspace API.  These file descriptors will be
able to be used against libcxl.

This exports those file ops for use by other drivers.

Signed-off-by: Michael Neuling <mikey@neuling.org>
Acked-by: Ian Munsie <imunsie@au1.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>