powerpc/pseries: Introduce option to build secure virtual machines

Introduce CONFIG_PPC_SVM to control support for secure guests and include
Ultravisor-related helpers when it is selected

Signed-off-by: Thiago Jung Bauermann <bauerman@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20190820021326.6884-3-bauerman@linux.ibm.com