apb: QOMify IOMMU

This is in preparation to split the IOMMU device out of the APB. As part of
this commit we also enforce separation of the IOMMU and APB devices by using
a QOM object link to pass the IOMMU reference and accessing the IOMMU registers
via a separate memory region mapped into the APB config space rather than
directly.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Acked-by: Artyom Tarasenko <atar4qemu@gmail.com>