powerpc/configs: Add PAPR_SCM to pseries_defconfig

This is a pseries only driver, it should be built by default as part of
pseries_defconfig to get some build coverage.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210416111209.765444-1-mpe@ellerman.id.au