ocxl: Add a kernel API for other opencapi drivers

ocxl: Add a kernel API for other opencapi drivers

BugLink: http://bugs.launchpad.net/bugs/1746988
Some of the functions done by the generic driver should also be needed
by other opencapi drivers: attaching a context to an adapter,
translation fault handling, AFU interrupt allocation...

So to avoid code duplication, the driver provides a kernel API that
other drivers can use, similar to calling a in-kernel library.

It is still a bit theoretical, for lack of real hardware, and will
likely need adjustements down the road. But we used the cxlflash
driver as a guinea pig.

Signed-off-by: Frederic Barrat <fbarrat@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
(cherry picked from commit 280b983ce2b8759722d911ea4b5af66e95d84e09)
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>