iommu/ipmmu-vmsa: Refactor ipmmu_of_xlate()

Refactor ipmmu_of_xlate() to improve readability/scalability.

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Link: https://lore.kernel.org/r/1611838980-4940-2-git-send-email-yoshihiro.shimoda.uh@renesas.com


Signed-off-by: Joerg Roedel <jroedel@suse.de>