powerpc/powernv/vas: Define vas_tx_win_open()

powerpc/powernv/vas: Define vas_tx_win_open()

BugLink: http://bugs.launchpad.net/bugs/1718293
Define an interface to open a VAS send window. This interface is
intended to be used the Nest Accelerator (NX) driver(s) to open
a send window and use it to submit compression/encryption requests
to a VAS receive window.

The receive window, identified by the [vasid, cop] parameters, must
already be open in VAS (i.e connected to an NX engine).

Signed-off-by: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
(cherry picked from commit 5239af679a07427647b009ebb9c70b1a03ebca9b)
Signed-off-by: Gustavo Walbon <gwalbon@linux.vnet.ibm.com>
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>