powerpc/pseries/vas: Define VAS/NXGZIP hcalls and structs

This patch adds hcalls and other definitions. Also define structs
that are used in VAS implementation on PowerVM.

Signed-off-by: Haren Myneni <haren@linux.ibm.com>
Acked-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/b4b8c594c27ee4aa6be9dc6dc4ee7331571cbbe8.camel@linux.ibm.com