powerpc/pseries/vas: Integrate API with open/close windows

This patch adds VAS window allocatioa/close with the corresponding
hcalls. Also changes to integrate with the existing user space VAS
API and provide register/unregister functions to NX pseries driver.

The driver register function is used to create the user space
interface (/dev/crypto/nx-gzip) and unregister to remove this entry.

The user space process opens this device node and makes an ioctl
to allocate VAS window. The close interface is used to deallocate
window.

Signed-off-by: Haren Myneni <haren@linux.ibm.com>
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/e8d956bace3f182c4d2e66e343ff37cb0391d1fd.camel@linux.ibm.com