powerpc/powernv: Honor the generic "no_64bit_msi" flag

commit 36074381 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: Jiri Slaby <jslaby@suse.cz>