powerpc/papr_scm: Use depend instead of select

Making PAPR_SCM select LIBNVDIMM results in circular dependencies in
Kconfig when another symbol depends on it. Fix this by replacing the
select with a depends.

Fixes: b5beae5e ("powerpc/pseries: Add driver for PAPR SCM regions")
Reported-by: Alastair D'Silva <alastair@d-silva.org>
Signed-off-by: Oliver O'Halloran <oohall@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>