sun4u: split IOMMU device out from apb.c to sun4u_iommu.c

By separating the sun4u IOMMU device into new sun4u_iommu.c and sun4m_iommu.h
files we noticeably simplify apb.c whilst bringing sun4u in line with all the
other IOMMU-supporting architectures.

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