powerpc/powernv: Honor the generic "no_64bit_msi" flag

commit 360743814c4082515581aa23ab1d8e699e1fbe88 upstream.

Instead of the arch specific quirk which we are deprecating
and that drivers don't understand.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>