powerpc/powernv: Use the security flags in pnv_setup_rfi_flush()

Now that we have the security flags we can significantly simplify the
code in pnv_setup_rfi_flush(), because we can use the flags instead of
checking device tree properties and because the security flags have
pessimistic defaults.

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