Move syscon_generic to attach much later

It still needs to be before if_awg at least in order to be available for
other operations, but it should not be attaching before interrupt
controllers at the very least.

This should make errors involving syscon register space colliding with other
devices a little more innocent, but these conflicts should really be tracked
down and resolved. One such conflict is with the Raspberry Pi 3 local
interrupt controller, noticed by tuexen@

Reported by: tuexen