powerpc/powernv/vas: Define helpers to init window context

powerpc/powernv/vas: Define helpers to init window context

BugLink: http://bugs.launchpad.net/bugs/1718293
Define helpers to initialize window context registers of the VAS
hardware. These will be used in follow-on patches when opening/closing
VAS windows.

Signed-off-by: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
(cherry picked from commit b25b33ac18b35775949ab227bb3075bb6cb11bc3)
Signed-off-by: Gustavo Walbon <gwalbon@linux.vnet.ibm.com>
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>