powerpc/pseries: Make plpar_wrappers.h safe to include when PSERIES=n

Currently plpar_wrappers.h is not safe to include when
CONFIG_PPC_PSERIES=n, or at least it can be depending on other config
options and so on.

Fix that by wrapping the entire content in an ifdef.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>