iommu/ipmmu-vmsa: Move num_utlbs to SoC-specific features
The maximum number of micro-TLBs per IPMMU instance is not fixed, but
depends on the SoC type. Hence move it from struct ipmmu_vmsa_device to
struct ipmmu_features, and set up the correct value for both R-Car Gen2
and Gen3 SoCs.