iommu/ipmmu-vmsa: Use refcount for the micro-TLBs

iommu/ipmmu-vmsa: Use refcount for the micro-TLBs

Reference-count the micro-TLBs as several bus masters can be
connected to the same micro-TLB (and drop TODO comment).
This wasn't an issue so far, since the platform devices
(this driver deals with) get assigned/deassigned together during
domain creation/destruction. But, in order to support PCI devices
(which are hot-pluggable) in the near future we will need to
take care of.

Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
Reviewed-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>