ocxl: Add AFU interrupt support

ocxl: Add AFU interrupt support

BugLink: http://bugs.launchpad.net/bugs/1746988
Add user APIs through ioctl to allocate, free, and be notified of an
AFU interrupt.

For opencapi, an AFU can trigger an interrupt on the host by sending a
specific command targeting a 64-bit object handle. On POWER9, this is
implemented by mapping a special page in the address space of a
process and a write to that page will trigger an interrupt.

Signed-off-by: Frederic Barrat <fbarrat@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
(cherry picked from commit aeddad1760aeb206d912b27b230269407efd5b06)
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>