sparc32_dma: use object link instead of qdev property to pass IOMMU reference

sparc32_dma: use object link instead of qdev property to pass IOMMU reference

This enables us to remove the last remaining (opaque) qdev property. Whilst we
are here, also update iommu_init() to use TYPE_SUN4M_IOMMU instead of a
hardcoded string.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Artyom Tarasenko <atar4qemu@gmail.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>