iommu/ipmmu-vmsa: Rework interrupt code and use bitmap for context

Introduce a bitmap for context handing and convert the
interrupt routine to handle all registered contexts.

At this point the number of contexts are still limited.

Also remove the use of the ARM specific mapping variable
from ipmmu_irq() to allow compile on ARM64.

Signed-off-by: Magnus Damm <damm+renesas@opensource.se>
Reviewed-by: Joerg Roedel <jroedel@suse.de>
Signed-off-by: Joerg Roedel <jroedel@suse.de>