powerpc/powernv: Enable tunneled operations

P9 supports PCI tunneled operations (atomics and as_notify). This
patch adds support for tunneled operations on powernv, with a new
API, to be called by device drivers:

pnv_pci_enable_tunnel()
   Enable tunnel operations, tell driver the 16-bit ASN indication
   used by kernel.

pnv_pci_disable_tunnel()
   Disable tunnel operations.

pnv_pci_set_tunnel_bar()
   Tell kernel the Tunnel BAR Response address used by driver.
   This function uses two new OPAL calls, as the PBCQ Tunnel BAR
   register is configured by skiboot.

pnv_pci_get_as_notify_info()
   Return the ASN info of the thread to be woken up.

Signed-off-by: Philippe Bergheaud <felix@linux.vnet.ibm.com>
Reviewed-by: Frederic Barrat <fbarrat@linux.vnet.ibm.com>
Reviewed-by: Christophe Lombard <clombard@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>