powerpc/powernv/vas: Define vas_init() and vas_exit()

powerpc/powernv/vas: Define vas_init() and vas_exit()

BugLink: http://bugs.launchpad.net/bugs/1718293
Implement vas_init() and vas_exit() functions for a new VAS module.
This VAS module is essentially a library for other device drivers
and kernel users of the NX coprocessors like NX-842 and NX-GZIP.
In the future this will be extended to add support for user space
to access the NX coprocessors.

VAS is currently only supported with 64K page size.

Signed-off-by: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
(backported from commit 4dea2d1a927c61114a168d4509b56329ea6effb7)
Signed-off-by: Gustavo Walbon <gwalbon@linux.vnet.ibm.com>
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>