iommu/riscv: Allow RISC_VIOMMU to COMPILE_TEST

iommu/riscv: Allow RISC_VIOMMU to COMPILE_TEST

This driver used to use a lot of page table constants from the architecture
code which prevented COMPILE_TEST on other architectures. Now that iommupt
provides all of the constants internally there are only two small bumps
preventing COMPILE_TEST.

- Use the generic functions for the riscv specific phys_to_pfn() and
  pfn_to_phys()

- Use CONFIG_MMIOWB to block off the mmiowb() barrier

- Require 64 bit because of writeq use failing compilation on 32 bit

Tested-by: Vincent Chen <vincent.chen@sifive.com>
Acked-by: Paul Walmsley <pjw@kernel.org> # arch/riscv
Reviewed-by: Tomasz Jeznach <tjeznach@rivosinc.com>
Tested-by: Tomasz Jeznach <tjeznach@rivosinc.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>